Ava loved Oaths.
She found it helpful especially when she was dealing with a more powerful individual. Breaking an oath could be difficult for some people and it would give her an assurance that someone would have her back in case something happens.
Call her paranoid but Ava would want to keep herself as safe as possible.
"Yes, an Oath that you are not going to harm me and that I am under your protection inside that Kingdom."
"Why would you need my protection, Miss Woods?"
"The same reason why Baba protected Matthew in secret?"
Ulva widened her eyes. "I see. You seemed to be aware of the past and the current situation inside the Kingdom."