Luc was jealous of Frimelda being a better swordsman than he was, and no matter how hard he trained he could not reach her level. So, he sought to use foul play in order to obtain her sword, the Fallen Angel, and be the greatest swordsman. To do this, Luc poisoned Frimelda with Zombie Powder. Clan Gully defeats him and Frimelda chooses to spare his life, not desiring to stoop to his level.
When Luc is encountered again later in the Frimelda Quest Line, the party fights with him as an ally against several zombies. Luc has seen the error of his ways and seeks retribution by means of defending Frimelda from the zombies who see her as an outcast.
After the battle and Frimelda is restored to her former body, Luc leaves and is never seen or heard from again, most likely still trying to repent for his misdeed.