(Note the description implies a full paragon path, alternate outcomes listed below.)
It is unknown where Kasumi was born or how she came to be a thief let alone the best thief on the galaxy, but some details are known. Early in her career she used to leave a red rose at all her crimes as a calling card though Keji eventually broke that habit by pointing out how overly sentimental it was. Kasumi had man daring raids and carefully planned heists along with Keji once they met though all that changed when Donovan Hock captured Keji and killed him for his graybox implant.
Mass Effect 2: Stealing Memory
When Cerberus rebuilds Commander Shepherd and the Normandy in order to investigate the Collector attacks on Human colonies, Cerberus officer Maya Brooks complies a list of dossiers and Kasumi is one of them. Cerberus tries to make contact with the thief, but has little success since she found out and began tailing them to see what they wanted. When she finally learned they wanted to hire her to help Commander Shepherd she accepted since Kasumi was a fan of his. She did this on the condition that Shepherd aid her in a theft of Keji's graybox. Kasumi meets Commander Shepherd on the Citadel and uses an advertisement column to speak with him. Kasumi then joins the Normandy crew.
Once they are on the shuttle ride to Ber Kasumi lets Shepherd know who they are robbing and what is going on. Donovan Hock is a high class weapons dealer, smuggler, and thief who killed Keji, Kasumi's partner, for his graybox. A graybox is an internal implant that stores memories and Hock wanted Keji's secrets. When it comes time for the heist against Donovan Hock Shepherd and Kasumi take a shuttle to Hock's estate where he is hosting a lavish party for mercenaries, gangsters, and other high class low lives. Kasumi fabricates a back story for Shepherd as a mercenary leader who operates a small, but effective team in the Terminus systems. Kasumi also arranges to bring a fake golden statue of Saren Arterius as a gift, but it contains their armor and weapons.
The idea is to breach Hock's vault while the party is underway and get away unocticed withe the graybox. Once planet side they hit a snag when Hock won't allow Kasumi inside, though this doesn't stop her from cloaking and going in any way. Once they locatable the safe Kasumi finds they need a suitable DNA sample, Donovan's voice saying the password, and to shut down the power to a force field. Kasumi and Shepherd are able to locate the power source near a fire place and disable it. For the voice lock Shepherd gets Hock talking long enough for Kasumi to program his voice while they find the password "Perguia" in the security office. For the DNA sample Kasumi and Shepherd sneak into Hock's private chamber and scan various items to compile a DNA sample.
Once inside the vault they see just how extensive Donovan Hock's collection really is. He has obtained the head of the Statue of Liberty, Michelangelo's David, statue of a Rachni Queen, Krogan statues, Turian artwork, Quarian stone tablets, A statue of a Darkspawn Ogre, and a pair of M-12 Locust sub-machine guns which were responsible for killing 2 Presidents. Once they find the graybox Hock proceeds to call in security who fail to kill Shepherd and Kasumi. The pair fight their way out to the landing pad where they encounter heavey resistance my Elicpse mercenaries and Donovan Hock himself inside a shielded gunship. unfortnatley form them its not enough and Kasumi jumps onto the gunship and disables the shiel;ds while Shepherd brings it down killing Donovan Hock.
Once aboard the shuttle Kasumi patches the Graybox to her own so she can see her parnter and now revealed lover Keji again. Keji as a master hacker laced the dark secret into his own memories and askes Kasumi to destroy the evidence by wiping away his memories. Kasumi is conflicted since she wants to keep this part of Keji and Shepherd lets her, but warns her to never lose the graybox. Aftger this Kasumi continues to serve aboard the Normandy and has a new found loyalty to Shepherd.
She helps in the assault on the Collector Base and survives the ordeal. After the mission Commander Shepherd is ordered back to Alliance space to face charges after accidentally killing 300,00 Batarian colonists. Because of this most of the squad including Kasumi leave the ship to avoid an Alliance interrogation.
Mass Effect 3: Hanar Diplomat
It is implied that Kasumi continued her thieving career after her time aboard the Normandy SR-2. It is noted she drew the attention of a Salarian Spectre named Jondaum Bau who sought to bring her in. They seemed to have a mutual respect for one another praising each others skills and tactics. Commander Shepherd will eventually be contacted by Bau who requests aid in finding an indoctrinated Hanar diplomat. This diplomat was exposed to Reaper tech during a Alliance black opts raid on a Batarian science team who discovered the technology. The Hanar have off the books trade with the Batarians so they were a great help to the Alliance for this raid. One of these Hanar is now a diplomat and Bau has tracked several suspicious transactions from Kahje to the Citadel.
Once Shepherd enters the Spectre office Kasumi de-cloaks and lets Shep know that this raid was the secret that her partner Keji died protecting. This info could have sparked a war between the Alliance and Batarians though their is little reason to hid the info now as Bau discovered the raid and the Batarians have already fallen to the Reapers. Kasumi claoks and follows Shepherd through the investigation often commenting on the situation. Once Shepherd has discovered the Hanar is a diplomat named Zymandis they meet up with Bau in the Embassy office. Zymandis expalis that he found information that the Collectors were the Reaper controlled Prothienas and since the Hanar worship Protheians like gods he reasoned that the only way to still enact their religion was to aid the Reapers. Zymandis uploads a virus to Kahje's automated defense system in order to make the planet easier to conquer.
Shepherd jumps to save Bau from another indoctrinated soldier while Kasumi stops the download though appears to die from a trap rigged into the console. Bau kills Zymandis and was surprised to see Kasumi was there the whole time. Shepherd explains she was a friend and died to save the Hanar homewolrd. Bau takes the point and promises to have Spectre's ready during the fight to retake Earth. Once he is gone Shepherd askes Kasumi to come out of hiding. Kasumi is surprised the Shepherd saw through her trick in order to get Bau off her back while still doing the right thing. Shepherd then recruits Kasumi onto the Crucible project figuring her knack for hacking alien technology would come in handy.
Mass Effect 3: Citadel
Kasumi is later encounter by Shepherd when she enters the Silver Coast Casino with the intent of robbing the palce. Once she sees Shepperd she tries to weasel out of guilt by saying the money can be better spent in the war effort or on refugees from conquered worlds. Shepherd chooses to not let the guards catch her or her team. When Shepherd throws a party of all the Normady crew past and present Kasumi is invited though does not party the usual way.
She can be seen digging through Shepherd's underwear drawer, playing on the guest bed, beefing up the apartments security, and usually de-cloaking just in time to comment during a conversation. Kasumi will play poker with James Vega and Liara T'Soni and comment on how Liara would make an excellent partner in crime.Kasumi unwittingly drinks some real milk under the impression that it is soy and spends some time in the bathroom thanks to her lactose intolerance. Well actually she is not lactose intolerant she just doesn't put up with lactose's stupid drama.
Kasumi does de-cloak for the massive group photo and comment son how much fun she had at the party the next morning. Kasumi will then join Shepherd in the Armax Arsenal Arena as a squad mate once her license is purchased. Once shore leave is over Kasumi presumably goes back to the Crucible project and Shepherd contacts her one final time before the final push on Earth. She says that if they survive she will go buy the drinks for the victory party.
Powers and Abilities
Kasumi Goto is the best thief in the galaxy though not the most well known, but she likes it that way. Kasumi is noted as a tech expert and can hack a variety of high grade system whether they are manufactured by humans or one of the alien species in the Mass Effect Universe. Kasumi in battle typically wields a sub-machine gun and heavy pistol and takes full advantage of her cloaking ability by attack enemies from behind or even setup a holographic decoy of herself to fool her enemies.
Mass Effect 2 - Kasumi's loyalty mission can be ignored or she may even never be recruited depending on Commander Shepherd's choices in the game
Mass Effect 3 - If Kasumi was never recruited then Commander Shepherd will have to choose between rescuing Jondaum Bau or saving the Hanar home world from Reaper invasion, in essence choosing either Hanar or Spectre support. If Kasumi was recruited, but her loyalty mission was never complete she will then die when the computer terminal goes off.