In two words: possessive jerk.
And please keep in mind that I actually like him XD Their relationship is mainly built on lust though later on they start to get being fond of each other, at least on Yui’s part.
He used Yui and her blood for his own personal gain so he can get stronger and be number one once and for all, tried to kind of burn the heroine with hot wax, used her for darts target practice, fucked her on a table in front of the others
Laito wanted to join XD;;, and put her on a leash and told her to act like a dog.
End A: He stabs her in the heart, tells her she is his forever while sucking blood out of her dead body.
End B: Breaks her mind and uses her as his sex doll/blood bank forever. At this point she can only utter broken “I love you”s.
End C: He marries her. That’s the only acceptable ending actually.
Ladies and Gentlemen, this is what Ayato is.
If I made any mistake please correct me.