Casey Jones

Casey Jones was a close friend of the Turtles, and dated April O'Neil.  His mother died when he was still in High school, and though his father, Arnold, became an abusive alcoholic Casey never fought back.  He had made a promise to his mother on her deathbed that he would take care of his father and he didn't want to go back on it.  But one time, during one of his father's episodes, Raphael happened to be walking by, and Raphael stuck up for Casey, beating his father right back.  Afterwards, Casey and Raph became friends, patolling the streets together, and Casey was there when Raph was finally reunited with his long lost brothers.

Casey stayed very close friends with the Turtles, and his getting captured by the foot is what started the City Fall story arc.  After the Shredder was defeated, Casey led the Purple Dragon Gang.