Does your version of Daisuke have any regrets about how he treated Sana? I’m rather curious
I haven't fleshed out too much about him yet, but I think I can answer this!
After Sana left, and her absence was noticed by the authorities (she's considered a missing person formally,) their parents began to harp on Daisuke more, since their usual bullying target of Sana was now out of the picture. Sana's mother gives her a hard time in her MSS about her grades being low, and in addition, the scene in the anime showing the whole Futaba family shows the older sibling (not Daisuke, the other one who I haven't given a name yet sorry) being praised for his exceptional grades. I took from this that academic success is praised by the parents, but academic failure is considered unacceptable. Daisuke himself mentions being accepted onto a sports team in the anime, so I figured that he's a sports prodigy. But, I've known sports prodigies in real life, and most of them had average or below-average grades, because academics just wasn't their thing.
I decided that it made sense that, while Daisuke excels at sports, his grades are only average, since he spends most of his time either practicing or playing. So, when Sana leaves, and her very low grades are no longer being noticed, her parents finally take a closer look at Daisuke's grades, and see how low his are, too.
Wow, this is getting long, I should make another full post with more backstory. But, the short of it is that once Sana leaves, Daisuke becomes the next scapegoat for the family, and his older brother, who was once his ally, begins distancing himself from Daisuke much like Daisuke distanced himself from Sana in order to avoid being abused the same way. He realizes that by avoiding a side in the fight, by remaining neutral, he had effectively helped aid in Sana's abuse. Daisuke's wish was to restore the name (reputation, that is) of his family, but after seeing his family return to the same abusive people they were before as a result, he realizes the error of his contract. From that point on, he vows to become a better person than them, and to not allow someone else to suffer as a result of his cowardly inaction.
Since Daisuke has contracted, he should be able to see Sana normally now. However, I imagine that the Futaba family lives in a different ward from Mikadzuki Villa (located in the Shinsei ward,) possibly in the Hokuyo ward (the upper-class ward that's home to St. Liliana's.) Since Sana attends Mizuna Girl's Academy, Daisuke wouldn't see her at school. I imagine that he attends either an unnamed all-boy's school similar to Mizuna, or one of the other schools nearby.
Thanks for asking about Daisuke, I really need to work on fleshing him out more!