Agumon is the partner of Tai the leader of the digidestined which makes Agumon the default leader of the digimon. Agumon first appears as Koromon, a lesser form of Agumons default form. He meets Tai and fallows him around as he explores the digital world.
When the digidestined first encounter a dangerous digimon their digimon companions evolve into their default forms and all attack the threat at once almost driving it away. Koromon evolved into Agumon and Agumon became his default form for the rest of the series.
Like every other digimon Agumon has unique moves and also has the ability to evolve into more powerful forms. In his default form Agumon can fire blasts of fire from his mouth which both burns and knocks the opponent back. He also has claws but doesn't use them often as his fire breath is a more effective weapon.
Agumon's more powerful ability is being able to transform into more powerful forms with the help of his human companion. Agumon has 8 other forms including he default "Agumon" form. His weakest form looks like a small black ball with eyes. His next form is his "Koromon" form and is not significantly stronger. After his "Agumon" form Agumon transforms into Greymon, a tyrannosaurus looking monster that can fire powerful fire blast from his mouth, powerful enough to leave craters where it impacts. His next form is called Metalgreymon which looks similar to his Greymon but with cyborg like body parts and is much more powerful. This form can fire rockets from his cyborg chest plate that can destroy buildings. Agumon's most powerful form is called Wargreymon and it looks like a humanoid tyrannosaurus with thick armor. This form can create spheres of energy which he throws at great speed at his opponents. These energy spheres are incredibly powerful, equivalent to a miniature nuclear explosion. Wargreymon is one of the most powerful digimon in existence.