* In the Manga

Zagine was a hired assassin who killed Train's parents. As he did not know they had a child, he didn't plan on meeting Train, and so gave him the chance to live. Train lived with him, under a strict and demanding training, in order to survive. It was during his childhood that Train got his signature characteristics: Drinking milk and developing cat-like reflexes. Zagine later died, leaving Train to be on his own, and told him that to survive in this world, he would have to be the best. That is when Train started improving himself in order to become "the best".

* In the Anime

Same as in the manga, except Train is influenced by Zagine when he reaches Eden, and confronts a "ghost" of Zagine. Nevertheless, he faces him and doesn't let his words bring him down.