According to the Warhammer Wiki, Gotrex and Felix are wanderers who offer to become a general and a hero for Empire, Bretonnia and Dwarf factions.

They appeared in my Immortal Empires campaign and offered their services to Tzarina Katarin's Ice Court faction, so it seems that Kislev factions can hire them too. Once hired in Immortal Empires, it appears that they remain in your service for 30 turns before moving on. Have you hired them - I'm wondering which other factions can get them - and how do you use them?