Gwen Tennyson originally appeared in Ben 10 as Ben's cousin, it was shown that these two did not like each other but as the show progressed the two started showing a deeper family bond.Gwen has prowess in three different forms of the Martial Arts and is also good gymnast. Apart from those things, she also found out that she could do magic.
In the series Ben 10, Gwen wore a blue shirt with a cat logo on her chest (However in Ben 10,000 Returns the cat was gone for some reason). Her lucky girl outfit is worn in Tough Luck and Lucky Girl.
During the "Change of Face" episode Charmcaster told Gwen that she possessed a magical aura which allowed her to cast spells when she had switched bodies with Gwen. During this episode Gwen stole Charmcaster's spell book and started practicing magic,with her first successful spell was when she commanded water to rise to stop a couple escaping convicts from swimming away. During Alien Force and Ultimate Alien, Gwen's mana and magic has grown much more powerful; which all derive from her alien grandmother Verdona, who is an Anodite.
Ben 10: Alien Force
Her powers and abilities are revealed to be of alien descent; her paternal grandmother, Verdona, is an energy being known as an Anodite; from the distant planet Anodyne, and surprisingly she seems to be the only human descendant who inherited Verdona's powers. Verdona tells Gwen that what she and others originally thought of as "magic" was actually the control of mana, a substance of magic-making Anodites; a magic-wielding race. Verdona offers to take Gwen to her home planet and train her in the immense mystical and magical ways of the Anodites, but Gwen prefers to remain on Earth with Ben and Kevin. She dislikes being called an "energy being", referring to her Anodite heritage as revealed in the episode Paradox. In the episode In Charm's Way, it's implied that highly gifted humans like Charmcaster can also be trained to utilize mana.
In Unearthed, an alien creature was able to see her as an Anodite, in which she looked exactly like her paternal grandmother's true form, but slightly shorter. In the two-part second season finale War of the Worlds when Kevin was threatened by a Highbreed Commander, she released her dormant innermost Anodite self for the first time, and easily defeated the commander with an exceedingly strong powerful burst of pure magenta-pink glowing energy. She was almost overwhelmed by her own power and wanted to defeat the highbreed all on her own before Kevin snapped her out of it by saying " I can't lose you".
During the course of the series she has developed a romantic relationship with Kevin. They have also been shown to have arguments, usually starting with Gwen always trying to force Kevin to "ask her out" and him always answering "Don't push me" (which she once did).
In the episode Save the Last Dance, Gwen has a dance in her prep school and expects Kevin to ask her out. The episode ended with Gwen and Kevin dancing together in the moonlight. In the same episode, it was revealed that she liked yellow roses.
In the episode Fool's Gold, Kevin appeared to be avoiding her because he was ashamed of his recent mutation due to accidentally absorbing a massive feedback pulse as a result of trying to hack the Omnitrix. She was unaffected by the blast because she put an energy shield around herself. Kevin gave her a locket of the two of them when he was in his human form, asking her to remember him as he was. Gwen said she didn't care what he looks like, and he replied "Maybe not. But I do."
Ben 10: Ultimate Alien
In Hit 'Em Where They Live, Zombozo and other villains teamed up to take revenge on the Tennysons and company, and kidnapped Ben's mom Sandra. Enraged, Gwen released her dormant, inner anodite for the second time since, War of the Worlds:Part 2. Whilst Zombozo trembled with fear, Gwen began to grow to a huge height, promising that if one of his teammates (and other notable villains) even bumps into their families on the street, she would come for them. Petrified, Zombozo screamed off-screen and Gwen saved Sandra. When Kevin asked what happened to Zombozo she simply replied "We came to an understanding" whilst smiling.
In Video Games, she tries out for her driver's license, earning it in the end. In the early episodes of the series, Gwen and Kevin seem to be bickering constantly. Gwen's magenta-violet-purple shields look less see through and have a different design than in Ben 10: Alien Force. Perhaps the color intensifies to much brighter shades of pink, magenta, and fuchsia as she grows stronger and more powerful, (evident in later episodes where she demonstrates her advanced powers and abilities.)
In the episode Hero Time, it is shown that Gwen gets very jealous when Jennifer Nocturne starts flirting with Kevin after he saved her life. And Kevin doesn't seem to mind. Kevin also protected Jennifer a lot, over the course of the episode. For example; When Overlord shot an energy blast at them Kevin put his arms around Jennifer to protect her from the energy beam, even though Gwen put up a magenta-pink shield around them. Gwen eventually lost her temper and threatened Jennifer that she would, "peel her like a grape" if she made another move on Kevin.
In Ultimate Aggregor, Gwen is seen to have much, much greater skills with mana. In Map of Infinity, it is revealed that Gwen's Anodite powers do not affect Galapagus' species and Aggregor, (with his powers), leaving Gwen vulnerable. From Map of Infinity onwards, Gwen starts casting spells and reciting spoken incantations again. She is also seen with a Plumbers' suit. She is the only one of the group that can use her powers in the suit, during her attempt to recover the first part of the Map of Infinity, she discovers it was hit by a poison dart and ends up fainting, causing Aggregor to catch the first fragment, but Kevin and Ben save her from near death.
In Reflected Glory, Gwen is comprehensive with JT and Cash. In the final part of the episode, Gwen is seen with her shirt torn and her hair disheveled, after she and Kevin face the RED's again. Gwen had the idea to use a mirror against RED's, and her powers apparently did not greatly affect the RED's.
In Deep, the group goes to the planet Piscciss to help Magister Pyke stop Aggregor. Gwen is caught by a squid-like creature, but Kevin revealed that he installed neuro-shock pulsers which can shoot lightning allowing Gwen to get free. When they reach the nucleus of the planet Piscciss an earthquake occurs, Gwen put an energy shield around them but failed to protect Magister Pyke, so she stays with him while Kevin and Ben go after the second part of the Map of Infinity. However, Aggregor arrived first and fled with the second part of the Map of Infinity after defeating Ben and Kevin. When the planet begins to crumble Ben as Goop saves the planet, re-constructing the lost water particles using Goop's artificial Gravity immobiliser. When Ben apparently dies, Gwen is devastated, but she sees that he survived, she feels immediate relief. At the end of the episode, Gwen consoles Kevin for having to leave the Little Fish in Piscciss.
In The Forge of Creation, Gwen travels with Ben and Kevin to stop Aggregor from absorbing a newborn Celestialsapien's powers. Young Ben comes along as Ben accidentally falls in the time barrier. Gwen, Kevin, and even Ben became annoyed with young Ben. However everyone eventually understands why Young Ben is so arrogant and cocky, when dealing with the pressure and the danger that could kill somebody, Gwen's first-cousin tends to joke around so he can be prepared to face anything. Unfortunately everyone was easily defeated by Aggregor, who seems to have won, but Kevin absorbed the immense energy of the Ultimatrix as a last resort, defeating him but becomes insane in the process.
In episode seventeen, ...Nor Iron Bars a Cage, Gwen and Ben travel to Incarcecon to warn the warden about Kevin. The warden would have been killed if it were not for Gwen, (who saved him from crash landing using a much stronger version of the "Turbo" spell.) Ben implies Kevin needs to be "put down", which shocks Gwen.
In The Enemy of My Enemy, Ben and Gwen are shown to be having difficulties. The two argue about Kevin while trying to locate a ship. On the way, they stumble upon Argit, who needed a place to hide from Kevin. They travel to the Plumber's Academy in the Rust Bucket 3 only to be attacked by Kevin. They barely succeeded in defeating him, and Gwen said she turned the air lock on to send Kevin falling because she understood he would kill them if he had the chance. After Argit preformed his orders, Kevin attacks the Plumber's Academy. Ben uses Ultimate Humungousaur to deal with the nuclear bomb while Kevin relentlessly attacked Argit. Gwen tells him to stop and to give himself up! But he sends her to the ground. Gwen remains there crying as Kevin spares her from his fatal blow, stating that she was only spared because of what she used to mean to him. Ben returns and the two have a conversation, Gwen says that they could still help Kevin, Ben replies that they cannot.
In Absolute Power, Gwen is more determined than ever to save Kevin. Ben disagrees with Gwen, opting to kill him now rather than a last resort. Gwen goes to Max hoping he will disagree about killing Kevin, but her grandfather confirms that killing Kevin is the right decision. Telling Gwen that if she disagrees, she should leave Ben to do it and not get in the way. Gwen leaves, disappointed and angry, ready to take desperate measures she confronts Michael Morningstar for help. Meanwhile, Ben goes off looking for Kevin, who's relentlessly going after anyone he thinks has betrayed him. Just as Kevin beats up Vulkanus, and begins to go after Ben, Gwen stops him, accompanied by Michael. Ben is infuriated by Gwen's alliance, but she makes a deal with Ben, and if her plan does not work, she would not bother him anymore in his decision. Gwen also admits that she promised Michael enough mana in return for his help that will keep him at full strength and power for one year, furthering Ben's anger toward her.
Michael explains he saved a part of the Dominus Librium which can be used to turn Kevin back to normal. But the machines and energy needed is only available at Los Soledad. In order to lead him there they need bait. Gwen suggests herself. Gwen goes to an abandoned arcade and confronts Kevin. He admits that he has been avoiding her so he will not absorb her energy, which he is desperately hungry for. They fight intensely, and Gwen tries talking some sense into Kevin. Unfortunately, this doesn't work and Kevin grabs a hold of Gwen and absorbs both her magical and Anodite powers, making her scream in pain. Gwen retaliates with a mega blast and escapes.
Michael and Ben travel with Gwen in the Rust Bucket 3 to Los Soledad, where Michael trapped Cooper to create a machine that could operate the Dominus Librium. Kevin goes to Gwen's house in search of more manna but finds Max. Another fight occurs, and just as Kevin is about to kill Max, Harvey, Kevin's step father, stops him. Ben appears after Kevin and Harvey's altercation and fights Kevin, yet again. They destroy the Rust Bucket at which point Gwen comes out. Kevin manages to disable Ben long enough so he can chase after Gwen. After a long chase, Ben returns and fights with Kevin once again.
Kevin defeats Ben, but little does he know that was all part of the plan. Gwen calls Cooper to tell him that Kevin will be there soon. Cooper tries to stall Kevin by using the weapons available at the military base, but Kevin dodges the missiles and approaches. Kevin and Ben fight with Ben no longer holding back. In anger he almost kills Kevin but Gwen stops him, as they could still bring him back. They finally set Kevin to the machine, and Kevin is healed.
Ben 10, 000
In the third season premiere, Ben 10,000, thirty-year-old alternate future Gwen (who has been going by Gwendolyn since college) brought Gwen and Ben twenty years into the future during Grandpa Max's 80th birthday. Her hair is now short.
In the non-canon episode Ken 10, an astral projection of a 42 year old Gwen was seen during Ken's tenth birthday just to say happy birthday to him and gift him a stone monster. She also seems to have the five Charms of Bezel in her possession though for an unknown reason, she does not have the Lost Keystone of Bezel. Her hair is long and worn in a ponytail. In both alternate futures Gwen has a close relationship with Ben and cares very much for him. In Ken 10, she is also seen behind him, saying he should let Ken have a chance.
Future Gwen was mentioned by Ben 10,000 when he told the present Ben that he'd become president someday with Gwen taking his position and to eventually get a second term.
Powers and Abilities
Mana Manipulation and Energy Control
Gwen can create energy blasts, shields and barriers. With Gwen's mana abilites her powers are mainly used like the power of energy constructing allowing her to make many different types of structures like, hammers, protective helmets, battering rams,beams to grab and throw enemies, stepping stones/platforms to carry her and others though air, saftey nets; turns out Gwen can also synchronize her martial arts styles with her energy manipulation abilities.
Magic and Spells
In Where the Magic Happens, Gwen claims that being self-taught in magic is not so easy as it looks. Trying to get into Yahwahtacsip she mistakes its name for "Wah Di Tah", saying that she knows how to read it but not pronounce it correctly.
From Map of Infinity onwards Gwen begins to cast spells and recite spoken incantations again. Gwen is less knowledgeable than Charmcaster and Hex, having been self taught, but she is also more powerful than both Charmcaster and Hex.
She has also learned to astral project herself and others through magic.
Athletics and Martial Arts
Gwen is a capable gymnastic and martial artist. Even in the original series, she was occasionally shown to be quite good in hand-to-hand combat fights against enemies who had normal strength levels. At the age of fifteen it is revealed that she has already received a black belt in taekwondo.
When preferring to use her athletics during fights or when she cannot rely on her magical and Anodite powers, she is shown to be highly skilled on hand-to-hand combat.
According to Dwayne McDuffie on one of his forums, Gwen does martial arts, horseback riding, softball, skiing and cross country. Turns out that Gwen can also synchronize her martial arts styles with her mana manipulating abilities. In the episode Ben 10,000 Returns, Gwen is shown creating a whip of mana energy and using it as a weapon along with her fighting skills. When Kevin asks her how she learned it, she states remembering it from an old movie.
Knowledge and Intelligence
Gwen is shown to be very intelligent and knowledgeable. Before discovering her dormant innate magical and Anodite powers, she would often use her "brains" to help Ben. She is shown to be a bit of computer and tech wiz as well. She has her own laptop which she uses to research enemies she meets.
In the Back With a Vengeance, Gwen switched the Omnitrix with a bomb, saving both Ben and the world, showing her cleverness.
In episode It's Not Easy Being Gwen, it shows that Gwen takes her education very seriously. Showing to come home late at all hours of the morning and having to wake up at 5:30 am in the morning. When waking up so early she does a number of things before going to school. Such as jogging, practice french, school, and afterschool karate classes. It's also presumed that Gwen goes to private school since it shown that her usual clothing are actually a uniform at her school.
Spells used by Gwen
Reverto Maeos Opsmihi- helps the caster regain their magical abilities and powers if drained from them.
Turbo- creates an extremely powerful air and wind spell that can be used in different forms.
Ortis(Exposits)- summons a spell book from a place to the caster's hand.
Quis quandum era mafilctus, iem fiechus mas redentegero, quis venit pro mas mesquam essei- creates a triangle- shaped portal that allows the caster to travel through time.
Abeo Exerior- places the caster and others in dome of mana and teleports them to a certain destination.
Eratico- sucks out the life energy from someone and then traps them in an unbreakable sphere of various shades of mana.
Somnus- makes a person fall asleep.
Acerbus- allows the caster to turn off very bright lights.
Thrasius- makes a diamond or crystal shield around the caster that can deflect projectiles, mainly lasers.
Facho Gravis- forces the person to be brought down to their knees by the forces of gravity.
Pontum Cartigo- brings arcade consoles to life and attack the opponent.
Tempestus Erectum- creates a strong artificial silver wind.
Tempestus Impaetus- allows the caster to summon a lightning bolt from a clear blue sky to fry the opponent.
Silicus Milez Sositatio- summons Charmcaster's rock creatures.
Adifishio Patentuia- fires multiple beams of glowing mana from the caster's hands but has no affect on Anodites.
Scribo Inencendia- animates a pen and causes it write on its own.
Super Theca- puts out flames in an instant.
Astendo Specialis- a spell that reveals secrets.
Statuea- emits a wave of pink mana from the casters hand that causes the target/s to become immobile.
All the spells Gwen uses are in Latin.