Kentaro/Vera
Kentaro started his research at Bellefonte before it fell than became a Guardian to continue it. You wrote that she extended the use of her ability since she became a Guardian. He could have been a part of that. He's the kind to have pushed her power to its limits just to see what could happen. He does not enjoy inflicting pain at all but she could have a thing against him because of that. I think Vera is one of the Guardians who could more easily antagonize him if you want to go there and it could be interesting.