Alright. Uh. This got long. So I'm putting it under a cut. Also a lore dump on Envi and Yagi in our... not-really-an-AU. Because it's kind of significant in understanding how important they are to each other.
Envi and Yagi were both created at the same instance in time. They were collectively the first of Satanick's subordinates, created on complete accident when he was young. After, uh, an incident, they both sort of just... popped into existence. In Envi's case, to hold the pieces of him that he couldn't handle holding himself. In Yagi's case, to almost act as the voice of reason that no one around him would be and he was absolutely not capable of being. With... knowledge that they didn't feel they should be able to know. Primarily about their creator and themselves. They also both popped into existence very differently.
Envi, we pretty frequently joke is basically the personification of BPD. His very first moment of consciousness was blinding rage with seemingly no purpose or reason. He holds a lot of the anger and hatred Satanick would have otherwise felt for Fumus, and developed a fiercely protective disposition towards Satanick very quickly. Which eventually evolved into an infatuation and obsession with him. He's emotional and impulsive and violent and it's very difficult for him to see folks in any way other than black and white; good for or bad for Satanick.
Yagi, on the other hand, is highly analytical and rational. In the beginning, nearly everything he did was based in logic. He's an expert at rationalizing things and seeing the world in the shades of gray it really is. However, he also tends to refuse to take accountability for any of his actions because he can usually explain them away. He's much better at shutting his emotions out and tends to seem pretty apathetic. Over the years, however, he's certainly... become more impulsive. He's never been as impulsive or emotion-driven as Envi, though.
They've always been each other's most trusted confidante. Since the very day of their creation. They often meet up for drinks together (they used to just go on walks when Satanick was still young because there was not a bar where they lived), venting their frustrations and seeking the understanding they've only ever found in each other (granted, it's usually Envi venting... Yagi tends to just. Solve his problems). However, Envi also has the tendency to exhaust Yagi. One can only listen to the same problem so many times. And he worries deeply about Envi's obsession. He knows it has the capability of pushing him to hurt someone Satanick cares for (in fact, it has, though Envi hasn't ever told him so) and, in turn, get him hurt.
The convoluted...ness started when they both got wasted one particular night. Envi was, as per usual, ranting about things. That time, about Satanick's propensity to self-destruct and how much it tires him. Yagi... was too exhausted to care very much. Think of your typical "oh no, that's so horrible," when they're not even listening.
Then Yagi just... grabbed him and kissed him. Mostly to shut him up. Because he was exhausted and can only listen to Envi rant about the subject of his obsession for so long.
One thing led to another and Sullivan found them behind the bar later that night.
(That night is actually the night we've been working on a mini-comic about lol.)
They're both too stubborn to be the first to bring it up. So it's never been acknowledged, and neither has the fact that it happened again. And again. And again... and eventually became something of a routine. Seeking comfort in the only being who either of them feels really understands them. Plus, it provides a very much needed distraction for the both of them from their chaotic lives.