Secret Love

Falling in love with your mentor's younger sister is bad enough, but when she's also the daughter of your boss and 6 years younger, then it's worse. And especially when she's only 12 and he's 18 when they first meet. He has no intention of doing anything bad to her, and he tries to stay away from her because he does not want to catch feelings for a literal child.
Then 4 years later, he meets her again when she's with her new boyfriend. He goes to her house that night, pretending he wanted to speak to Viola -his mentor- but instead, he talks to Crystalia. And he blurts out how he's in love with her, and he doesn't want to see her with other guys. Then she kisses him, but he pulls away because he's an adult and she's 16. He begins to avoid her, but Viola finds him and tells him to come to Crystalia's 17th birthday.
Then she's of age in the wizarding world, and they start dating. The only problem is that he could lose his job if Viola or Andrei found out.