from what i remember he promises to marry the girls in the party ( or at least some of them ) once they grow up in a moment of weakness
(and months of pestering from a certain girl with mind control powers )
tho a lot of it was also down to getting the other sister some confidence in herself, She believes everyone in the world( him included) hates people with black hair when it just so happens to be his type
bad news bears however, she turns out to be a yandere once she do get that confidence.
She dont actually show that to him as far as i know, but when talking to people who she perceives is getting in her way ( of finishing her mission /meeting him again as soon as possible )
she becomes a pretty scary lady and does some pretty crazy stuff when she is on her own or only with her sister.
( the fact she and everyone in the party is super strong by then from MC training them while traveling sure doesn't help making her any less scary )