Finally got a chance to see the last big Flash. Here's what happened, as I understand it:
LE, who is THE BEST AT GAMES, has studied the events of the other sessions, and has learned enough about the game to 'sploit the hell out of it. He knows that, due to his/Calli's unique biology, the game will treat them as two players in one body, releasing their sprites at the same time and causing them to prototype each other in an infinite recursion. There are still 2 sprites, but they're Green and Red mirrors of one another; where LE's sprite is the Red Black Hole devouring the Cherub homeworld's supermassive sun, Calli's sprite has become the Green Sun at the other end; an ever-growing mass of matter, and a source of near-infinite energy and power. Pretty sure it's a metaphore for Calli as creative/a creator, and LE as a destroyer... but that's beside the point. Also beside the point is that LE, with his psychotic Lil'Cal-like chuckle, is the Mirthful Messiah whose coming Gamzee's religion foretold... hence why Gamzee is complicit in his rise to power.
Anyway, LE enters their session, and using some kind of Super Time Power exploit gets enough grist to alchemize
all his gear using Calli's sprite itself, the Green Sun. This makes them (and by extension him) infinitely powerful, due to the Green Sun constantly growing from stolen solar mass and infinite recursion. He also goes on to alchemize the First Guardians (via their special ectobiology codes) using the Green Sun, linking their powers to it as well; note that the First Guardians have various flavors of Uber Space powers (Beq teleporting, Doc Scratch's Omniscience, etc.), which are Calli's role's trademark. It's plausible that the Green Sun equipment grants LE access to Calli's powers as well, so he is able to get the best of both worlds, despite not having the role himself.
Despite his gaming competence, LE is clearly pretty unhinged... and judging by how much he's grown, and the head-twitch when he shows up to FIRE HIS LAZOR, he's spent a
very long time playing this game. He's sacrificed everything to be able to win at this game, and win harder than anyone else. He personally orchestrated the events that led the Trolls, Alpha Humans, and Beta Humans to this 4th Null Session, all so he can show them up at SBURB. As scary as he may seem, realize that he's akin to that neckbeard who devoted his entire adult life to mastering Ultima Online, and has made it his goal to grief everyone.
Also, he's still bound by the limitations of a game, and most importantly by his biology; Roxy knows his true name, and can wake Calli from inside him at any time. Of course, when a phenomenally-powered Calli wakes up, the kids might have to deal with an evil entirely different than the ubergamer that is LE; a supremely powerful ship-happy fangirl now living in her favorite story... I mean universe... with no living author to stop her from imposing her will on everything. It only makes sense that she's going to be the secret Big Bad.