Ok so from what i get

The story is set in 3 points in time, im telling them chronologically.

Back then, Metatron was hiding from (what seems to be) cherubims in the Garden of Eden, when he suddenly met a snake. He gave this snake his pendant. Hoping to let it escape, as snakes were not welcome in heaven. However, the snake followed him all the way home, and Metatron even ended up using it as an excuse to get away from one of his admirers. Later on in that same day, Metatron holds a small banquet with his lovers, and ends up meeting Beelzebub, who tries to entice him into having sex with him—Metatron refuses though, again, using the snake as an excuse. Later on, he even gives this snake a blanket to disguise it as a sheep.

That story just now was the origin of how the two first met, from what we've seen so far, it seems like Lilith, the Queen of Hell, helped Mammon(the snake) to evolve from a snake to a "fake seraph"— after which he lost his memories and turned into a child, living in hell getting bullied by other demons. After that comes the events from chapter 1 onwards, where Metatron makes a bet with Gabriel to make the seraph in hell(Mammon) fall in love with Metatron in exchange for sex. Metatron then goes to hell, and once he sees Mammon's situation, he gives Mammon a bit of his power as an angel to put judgement onto demons, which Mammon ends up using against his bullies. However, things go wrong when Mammon loses control of himself and ends up attacking Metatron using the borrowed power. Metatron tries to subdue and kill him, but finds that there are remnants of his own power in Mammon— remnants that shield Mammon and give him an opportunity to ultimately land an attack on Metatron. The thing is, Metatron himself has no memory whatsoever of giving this power to Mammon, besides from the power of judgement ge lent to Mammon, he gave nothing else.

At the end of the fight, Mammon transforms into a grown figure, and Metatron gets a glimpse of this. Later on, Metatron wakes up in the cave injured, and Mammon has transformed back into a child. It is clearly evident however, that now, Mammon has the ability to consciously transform into a child or grown form, but we have yet to know if Mammon already has his memories here— but it's clear he is already starting to realize his desire for Metatron.

Additionally, during Metatron's absence from heaven, the Heavenly Father has left, and instead, entrusted his position to Lucifer. It is implied that Lucifer and Gabriel have a deal, perhaps involving Metatron— and that Beelzebub is harboring suspicions towards Metatron for his past involvement with a snake. (Making it clear that while Metatron doesn't remember the snake, Beelzebub does.)

We then get a cut to the future, where Metatron is fully captured by a fully grown Mammon. Mammon is now the ruler of the ring of greed, and serves directly under Lilith, on this day, he is to welcome a delegation coming from heaven. Mammon has now recalled all of his memories as a snake, but Metatron still has no recollection of him, leaving Mammon to suspect that someone tampered with Metatron's memory. Thus, he goes to Lilith to find a way to peer into Metatron's memories, and she tells him that for it, Mammon would have to suffer a great deal of pain, to the point he may even want to die. Mammon pays this no mind, and welcomes the delegation from heaven, led by Raphael– Metatron's most trusted angel before he left. Mammon takes Raphael to see Metatron, Metatron waits inside of a room, holding a sharp brooch, ready to stab Mammon in attempt of an escape. However, he gives up on this attempt once he sees that Mammon brought Rapahel along, as he is afraid to drag Raphael in. Mammon proceeds to play a game with Raphael, where they can bet whatever they want from their opponent— whether that be an answer to a question, or a possession belonging to the other. Metatron agrees to be their referee in this game.

Raphael wins the first round, and takes Metatron's brooch(the same one he planned to use as a weapon) and dissolves it due to it already being cursed with demonic energy by Mammon— this is where Metatron realizes that Mammon knew of his escape plan all along, and that even if he proceeded with it, it wouldn't have succeeded either way. Raphael and Mammon tie on the second round, where Raphael asks whether the Heavenly Father is in hell, and Mammon answers no. Mammon also asks when Metatron first came to hell, and Raphael also gives his answer. Metatron tells them to end the game, however, Raphael insists on another round, saying this time, he wants Metatron back, and Mammon says in response that he agrees, but he wants Raphael's soul (He wants to use it to peer into Metatron's memories). This is where it ends so far.