Magellan is the antagonist hero of the One Piece franchise. He used to be the chief warden, but after he failed to prevent the jail's one and only mass-breakout, he was replaced by Hannyabal and demoted to vice warden.
Magellan is the chief warden of Impel Down, the most feared and impenetrable dungeon in the One Piece world. As such, he is a very imposing man with a very stern, business-like manner. Even when things get out of hand, he keeps up his professional manner, inspiring hope and admiration in much of the prison staff.
Magellan dedicates his job as the warden of Impel Down. He intends to put his poisonous abilities to good use to keep the most dangerous criminals in line. He is quite brutal, but still fair and reasonable; the only reason that he's even coming into conflict with Luffy is because Luffy's goals (freeing Ace) just so happens to conflict with his own (keeping order and making sure that a bunch of incredibly powerful and dangerous criminals stay locked up). In general, Magellan happens to guard the prison of Impel Down in hopes to prevent all of history's worst criminals from endangering the general public.