It will have to be Bell considering Ais is terrible with that kind of stuff. But I think Ais tells Bell no only because of the way she is (broken) mentally. I think she gives him a round about answer and Bell will ask her if she will give him an answer after the OEBD fight (that’s all my theory and opinion on the matter).
Or they actually do get together before OEBD (maybe even date in secret) and have a “love night scene” before the fight.