Knight by Charity Parkerson

Knight lives by his name from the military to taking in a friend in need. No good deed goes unpunished.
It started when an adorable pink-haired sprite slipped into Knight’s life. Before that moment, Knight hadn’t realized how much a person could love their best friend. Unfortunately, that friendship has him knee deep in the world of crime. Even more unfortunate for him, this predicament has Knight incapable of staying away from the most dangerous man he’s ever met. It might mean his downfall.
Royal had his emotions stripped from him years ago. He’s been trained to do one thing: kill. For now, he’s doing what needs to be done to survive. That means living with a family who thinks he can be redeemed. That ship sailed a long time ago. But now there’s this ridiculously hot art teacher who’s in over his head and Royal can’t stop thinking about him. Several times he’s considered killing him so he can move on, except he can’t. That’s never happened to him. It’s a puzzle Royal has to solve before the clock runs out and Knight learns the truth.