The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat S01 E03 Magic of Bonds

Lugh's magic teacher is a pretty girl named Dia. Being a few years older than Lugh, Dia treats him like a younger brother and sneaks into his bed every night in order to sleep next to him. Because he needs her to teach him magic, Lugh can't complain, but gradually he and Dia grow closer--