Marius is a high-ranked Dragon Slayer encountered early in the game. It is possible to have conversation with him outside Farglow, he will then tell you about his victory over the Dragon Knight Geldor. After becoming a slayer, the slayer will go to Broken Valley alongside Rhode and Marius in search of Talana. The slayer will eventually be forced to fight and kill Marius outside Maxos Temple after becoming a dragon knight.
You may encounter a woman named Alina in Crow's Nest who says that her mother encouraged her to marry Marius. She eventually married the Champion Servus instead, but seems to regret that choice. She calls Marius "a real man."