Zero no Tsukaima, Tate no Yuusha no Nariagari Recommendations
In both series the MC were summoned into another world. Both are Medieval-ish with Magic settings. Saito is like a slave for Louise in Zero No Tsukaima while the MC in Tate no Yuusha no Nariagari is the opposite. (Master). In the case of Saito he was summoned into Louise's world to protect her while Naofumi were summoned to protect another world.