The hero Heinrich, found in the alternate world accessed via the fourth Tartarus, he joins the party if you are on the Ilias route. Oh, and that strange mage he's with? Definitely just an ordinary human mage.
Heinrich is only recruitable on the Ilias route, being mutually exclusive with Alicetroemeria. During the quest for the Six Orbs, you will be directed to enter the world beyond the fourth Tartarus to obtain the Blue Orb. You need to use a ship for this, but the mayor of Succubus Village won't allow you to use their ship that easily. Go to Heinrich in the inn and Ilias will talk him into joining the party.