Board Thread:General Discussion/@comment-27524978-20160210030742/@comment-43574-20160331233254

He has some awareness of the different timelines and some memories from previous ones, but not "complete" like Flowey or the player. On Genocide he counts how many times you fought him up to a point. But he also mentions that he has trouble keeping track of those memories after a while, talks about knowing things "other-Sans-es" don't and states that sometimes his memories can reset completely. I guess the characters (apart from Flowey) can to some degree remember what happened in previous timelines, but very fleetingly, like a dream. Sans might just be more conscious of this. Flowey's dialogue at the end of a Pacifist run also imply that Chara has some control over what characters remember and what they forget.