When Sonic the Hedgehog, Miles "Tails" Prower, and Knuckles the Echidna arrived at a locale Chao Garden, they started to interact with the Chao they met there. One of the Chao there ended up inheriting Tails' flight capabilities, prompting Sonic to named it Chails.
Some time after, Dr. Eggman showed up under the assumption that their were magical items in the area. He would be disappointed when finding out there was not. Thus he went on to destroy Chails' home, forcing it and the rest of the Chao to move to the Lake of Rings.
Like most Chao, Chails displays a cheerful and welcoming nature to Sonic and his friends.
Powers and abilities
As a Chao, Chails can gain the traits of creatures to which it is exposed to as it grows, like when it took on Tails' traits. Once Chails nears the end of its life-cycle, it can envelope itself in a cocoon and emerge as an egg—reborn as a new Chao with the traits it had in its past life. It is as well capable of flight.
Having taken on Tails' abilities, Chails possesses greater flight capabilities than the average Chao.