It’s been insinuated - for example by Jasprose - in the comic that somebody could “remember” their doomed lives if they were able. And Rose sort of did that time. So I think the “ultimate self” refers to the like... over-soul that every self and timeline is a part of? Like even if you weren’t a part of it it was still *you* that died in a previous timeline. Just different veins in one big brain. And I guess sprite^2s are equipped to handle the knowledge of many veins. Blitzed chakra ghost guides

Pretty much. Not insinuated, all of that is pretty much explicitly mentioned by Davepeta, about the Ultimate Self at least. Rose remembered ONE life though, and still it was fuzzy fragments of her memory because of Dream Self Shenanigans, but yeah I get you.