I'm not good with these things, but I tried anyway! He is one of the antagonists, Kieran, and she is Christina, one of the protagonists.
-Kieran Óntuneric, pureblood wizard born in Romania who was Christina's best friend until he discovered that she is a half-blood.
He is two years older than Christina and he was in love with her when they were kids, but due to the strong pressures that his parents, who helped the Death Eaters after Voldemort's return,ended up hating Christina and her "dirty blood".
His parents moved to London when Kieran was 8 years old because his father wanted to join the Death Eaters but despite living in the UK, his father insisted that Kieran was sent to Durmstrang where all his family had studied, ('cause Hogwarts admits students with "impure blood" and Durmstrang is only for purebloods.)
Kieran's older brother Adrian decided to work for the Ministry of Magic as an Auror, going against his family and becoming an enemy, while Kieran and his sister Kathryn became Death Eaters and decided to stay in London after his parents returned in Romania.
He thus had the opportunity to meet important wizards who, like him, had anti-Muggle ideologies and therefore decided to be the new DADA teacher, like his new friends had suggested, in order to mold young minds to his will.
Despite being convinced of its superiority to Christina, he feels that something inside him is still in love with her and that's why he hates her so much.
Sorry if my english is bad xD