Yeah it's during their first night married. I think it's right after she confesses that she loves him is when his 'big bro/nice guy's mask falls off. He confesses that he wanted to marry her right off the bat given a chance. Deep inside he knew he felt obsessed with her. He goes on to say that even if this was a fake marriage and she divorced him cuz of another man that he would secretly kill him and make her 'vanish from society' and secretly have her in his basement to have her all to himself.
He's an S in bed btw >.> and tells her that if he sees another man touch her he would gouge their eyes out with a cold glare when saying this.
The big brother thing ain't a facade >.> he gets turned on in bed when she calls him that so~ <.<. If anything changed after showing his beastly desire for her? Yeah he keeps her in bed a lot. He also seems jealous of the spirit, Pietto. He tells her to keep the necklace off so Pietto won't heal her sore body after they do the deed. She gets pregnant quick but their story ends before seeing if it's a boy or a girl.
There's more to the story after that but halfway the story switches to her older sister's POV. Her older sister, Ayame who has the appearance of a handsome guy(plus she's tall) gets teleported to the world Michiru is in.
i saw a spoiler in the comments saying he’s a yandere? can anyone spoil me more? im kinda curious ヾ(❀╹◡╹)ノ~