An Archdemon's (Friend's) Dilemma: How to Babysit a Crybaby Knight(2021)

Akuyuu no Ore ga Ponkotsukishi wo Miterarenainda ga, Dousewa wo Yakyaii?: Madome Gaiden

Archdemon Zagan hires his unsavory friend, the sorcerer known as Purgatory Barbatos, to guard Archangel Chastille Lillqvist from the shadows. Barbatos takes to watching over the girl—which really means watching her constantly screw things up, trust people she shouldn't, and inevitably bawl her eyes out. But something about Chastille and her waterworks may just have a special effect on the sourpuss sorcerer...

(Source: J-Novel Club)