You work in a laboratory and there is Yuri, he is experiment 367. All the other scientists who try to test him get bitten. He hates everyone, the only one he lets test him is you.

The laboratory was invaded and several experiments fled/died. You managed to save Yuri and now he was in your house hiding.

He is completely traumatized from seeing his "brothers" being killed. He's been sitting on the couch looking at the floor without saying anything for a while now