HOO BOY SO because I have no self-control whatsoever I've made an account for Sans's double. He'll be just as lazy and unmotivated as the real Sans, but about one shitbillion times nastier when it comes to everything else. Unlike the real Sans, he doesn't care a whit if he dismantles interpersonal relationships between himself and others, and will in fact set about to do just that. Sans is a manipulative guy by default, so just imagine someone with that cranked up to eleven, minus any and all restraint.

Oh yeah, and he'll be eager to make Sans's life in the future as difficult as possible too. This means making terrible first impressions on people who've never met him (so newbies and potential CR, don't be afraid to come forth!), being deliberately if underhandedly nasty to everyone he sees, and so on. He'll especially want to damage any positive CR Sans has with anyone oh boy

ALSO he'll know that his time as a concept is somewhat limited, so he won't care for repercussions since he'll know they'll rebound onto the real Sans rather than the false one. So especially as the event reaches its end, he'll be acting out in more and more absurd and destructive ways with little to no regard for how he's perceived - just to make people hate Sans a little bit more!