Even after Rian thanked Ainsworth, he felt that it wasn't enough to express his gratitude so he added:
"I owe you one, Ains. Even if you want me to assassinate the Reigning Empress of the Valtoria Empire, I dare summon my light sword and cut her in half!"
If they were on Earth and other people heard what Rian had said, an SS-rank Awakener from the Valtoria Empire would have already been sent by Elyndra's mother to kill him.
It was because of knowing that Ainsworth cared about Elyndra that Rian dared to promise to do such a risky thing for him if Ainsworth ever needed him to.
Whether it was assassinating Elyndra's sisters or even her mother to let Elyndra succeed the throne, Rian dared promise him and keep his word.