The girl is too annoying. The guy is handsome and good so what does it matter that he is a bit different? She has been saved by his power before, so she has no right to call him a monster.
I honestly think that is a normal reaction. This whole situation is being forced on her. They really are not that good, they are both kind of jerks. I do think monster is a little to far, but she wants to hurt them because she is feeling powerless. I also don't think she realizes how much it is affecting them. To her it's like calling someone stupid. It's meant to hurt but only to certain degree. Non of them is taking the other into consideration.
Yuu is really sweet, but his actions this time are not acceptable! He is not showing any consideration for Jin's feelings. It made me feel a bit better that he cancelled the bet, but I really feel that Jin deserves better.