Severus snape was in love with Lily from a young age but when he was a teen he accidentally called her a mudblood and she hated him. And James and his gang humiliated snape throughout hogwarts years. But the worst incident was when they called him snivillus and pulled his pants down while hanging him upside down with the levicorpous spell. And you yelled at them and they ran off and you comforted him and he latched on to you and became close. You two started dating and protected him from James. You found him attractive and he gave up his love for Lily for you. But in 7th year you moved away making Severus depressed. One day when Harry was in first year you were a new teacher for hogwarts when you recognize snape. He recognized you and he wanted to go to you so bad but he didn’t want to show emotion in public. You are the new defense against the dark arts teacher. You are the same age as snape.