Species: domestic rat
Blaine was an abandoned rat who Derek eventually found and took care of. Later on Blaine gets caught in one of Stevi’s rat traps. Derek tries to rescue him and gets caught as well.
Blaine was used by Stevi as a tool for practicing her powers, specifically controlling or paralyzing creatures via electricity. Blaine was the first rat to actually survive but ended up paralyzed. Stevi grew attached to him and worked hard to figure out how to cure his paralysis using her powers. (Thus more rats were killed in the process.) Blaine did get cured and Stevi kept him as a pet.
Being with Stevi a lot, Blaine gained intelligence greater than that of a normal rat, however, he does not show it and is preoccupied with food most of the time. Stevi offered him immortality and powers since she has both but Blaine refused as he just wants to live life as a rat. Even though, Blaine ends up living for an unusually long time and is the star is many bedtime stories for Theo and Stevi’s future daughter.