you ask this question and then you hear a rumbling noise that grows steadily louder. you look up and just then i burst in through the wall kool-aid man style, screaming, with huge bundles labelled "my opinions on saimon" under my arms

the universal rule is this: the first character in a series who makes me shed a tear is my favorite for life. with this in mind, it is likely only the fact that MEMORY came out after LIVE that keeps saimon from being my favorite for life, because whenever I want to feel like throwing up a lung I just think about Those Scenes in MEMORY for five minutes. case in point thinking about those scenes in MEMORY for long enough to write this response did in fact make me want to throw up a lung

the thing about saimon that elevates him to me isn't just that he is way, way funnier than people give him credit for (but he is, btw), but the overarching theme of kindness as legacy, and a way of preserving the memory of someone you loved

it's the living through perhaps the most painful loss a person can experience and instead of letting it harden you for good -- and it is painted in broad strokes in MEMORY, but we know that the loss of Tsubaki made him spiral into nihilistic alcoholism for two years -- making a resolution to honor their memory through kindness and love.

so tl;dr my favorite thing about saimon is the last line of the First Note blurb for tcw:
so that the light you give to me may reach someone else in turn.
((jakkal go listen to kenshi yonezu's lemon and think about saimon and try not to throw up a lung challenge (failed). even now you remain my light))

anyway. i am not going to make any hard predictions for Karma, but based on the BoU opening show drama and the recent Animedia interview, I think we're looking at a case of Saimon feeling his age, or at least the age difference between him and a lot of the cast, which I'm very excited to potentially see play out! my only complaint about saimon as-canon is that he should be at least 10 years older, frankly.
there's also the fact that he feels he has an obligation to carry forth tsubaki's wish, and the fear that he may be failing to do so (again, possibly with the worry about the passage of time), which plays interestingly with the proposition amprule are going to be bringing in.

(i think people tend to over-simplify the idea of tsubaki's sacrifice, because it's not just "oh she died for nothing so saimon should hate phantometal" but i do not have the time or inclination to get into it now lol)