Kilorita The Victorious.

She could have tried to control him and it would have worked because he was in desperate need of her love and attention. She didn't because she didn't want a powerful puppet. She wanted him dead. 

She could have schemed to kill him and it would have worked because she knew a lot about him including his strength and weaknesses. She didn't though. Because his death wasn't enough for Kilorita. 

She didn't kill him even though the mere fact that he was already planning to remove the seal that she used on the planar portal was more than enough for her to want his death.

It would not have been difficult to kill him had she tried to. He might be the strongest but that is only for a brief moment when he uses the boost from his title. But she is better than that so she made him hide away instead of risking herself.