its suddenly occured to me that as soon as Simon Petrikov makes an official return as feeling like a member of the main AT cast in his own right, Jake is no longer present. It's still unclear how much of it is Jake having retired prior to his death, or if he was already dead by the events of Obsidian, but it feels like Simon has taken up the role Jake originally occupies.

Essentially when you look at Simon's events in Obsidian and the apparent role he seems to have among the bigger Finn-PB-Marceline-Simon party and group dynamic, he shares a LOT of surprisingly similar traits with Jake:

EXPERIENCED PARTY MEMBER: Jake isn't the oldest member of the group by any stretch, but the other characters TREAT him like he is; he's generally considered to have the most life experience, and he is often treated as being the wisest one. How much of this is objectively true is up for debate and a question for how much of it is just character viewpoints coming in, but Simon definitely has the most life experience. It's arguably one of his more defining traits, since having so much of it against his will is one aspect of the crown. Simon is also a pretty dang smart and emotionally intelligent person, though he doesn't acknowledge it.

THE DAD GUY: In the sense of 'being an older guy that the other characters talk to for advice and reassurance that can be relied upon to solve problems'. Jake isn't precisely a father to Finn in particular by any means, but he IS an older brother who can sometimes come off as having raised Finn, and he is definitely a dad. (Admittedly only for... well, a day because the Pups age weird, and it clearly bothers him, but it still counts.) As for Simon, him being a Dad is probably the thing that most hammered down his characterization and redefined him to such a colossal extent that Simon Petrikov overnight went from being the tragic origin of the Ice King to 'HOLY FUCKING SHIT HE WAS MARCELINE'S REAL DAD!!!!!', with all the implications of his character since. Simon is a dad, and his role as a dad underpins so much of what he seems to value and want more than anything else.

HYPER COMPETENT IN SPECIFIC FIELDS THAT FEEL MORE MUNDANE THAN THE OTHERS: Finn hits things and does Dungeon Adventure stuff. Princess Bubblegum is practically a text book mad scientist alight with the maddening light of GENIUS ITSELF. Marceline, interestingly, comes off very heavily as a parallel to Finn and Jake as a combo package, but the thing that characterizes Jake and by extension Simon for this argument is that they don't do the fancy stuff the other characters do. Jake hits stuff with his magic powers, but in practice he's a pretty blunt force person who doesn't do much of anything outside that, and Simon is notable in that he comes off as being kind of a useless weenie who THINKS he's a useless weenie... until the call of danger comes, and he abruptly shifts into overdrive and almost effortlessly solves the problems he faces without hesitation or any self preservation. In Obsidian he arranges a big calvary team completely on his own, off screen and without prompting, and in Fionna And Cake we see more of this. Simon THINKS he's useless while Jake doesn't care, but both of them are startlingly hyper competent in the narrow field of 'its kinda boring compared to the others but they do it'.

MORAL AMBIGUITY: This is probably the most potentially contentious one, but bear with me. Jake has an established history as a thief and general Dude Doing Crimes, and its not particularly clear if he stopped because he regretted it or just because he became more responsible for Finn. He doesn't SEEM like he regrets it in the slightest, either. Throughout the series it's a running theme that while Jake is generally wiser he's also way more prone to saying messed up stuff or suggesting iffy things for comedic purpose or just on a whim; taken seriously, it can feel like Jake is straight up Chaotic Neutral without much interest in the ethics he does, just about caring about specific people and not much else besides that.

Now let's look at Simon. The current fandom vibe is 'academic weenie with a bubble butt that screams in danger a lot', and this IS technically true, but its not the whole story. Ice King WAS a villain, and genuinely the most recurring threat throughout the series, and while that was part of the crown, the other wearers show it doesn't come out of nowhere. The potential to be Ice King, the callous and destructive brute force, is part of Simon in a whole 'there's two people in the mirror, the one you see and the one you DON'T want to see' kind of way. But during the course of Fionna And Cake we see Simon abducting someone he hates to use them as a magical battery and when they die from it he's more upset about losing his chance and not that someone DIED, when he watches a parallel version of himself wither away he brushes it off as 'oh they were a bad Simon'. It's not implausible Simon did some... STUFF back in the days of the post-war world to ensure he and more importantly Marceline survived. He has a history of revealing that underneath his weenie exterior, he's a solid mass of cold metal that's like biting on tin foil.

All in all, it feels that as Jake has implicitly left the narrative, Simon has been neatly dropped into fulfilling roughly the same role Jake originally did to the rest of the group, or at least that Jake's general narrative purpose of 'does more wacky things for giggles' has been generally split between Finn and Marceline, while Simon has taken on some of his Experienced Family Member and Moral Ambiguity bits.