Mukuro fucks him because they're outsiders, Irie Shouichi and he. Mukuro fucks him partially because he can, because no one is going to stop him, Irie is willing, and Mukuro is careful. His cute Chrome expressed concern, possibly confusion, but backed off quickly enough. She doesn't understand, but she tries, and it hurts something inside of Mukuro that he has become so reliant on people that he can stand Chrome's pity, her attempts at caring. He and Irie are outsiders at the Vongola, the scientist and the ghost, both of them unable to escape.
Sometimes Mukuro fucks Irie while wearing the body of another, a body pieced together from shared dreams with Chrome, from memories she has allowed him to access.
Chrome's mind is not as open to him as it once was. After her disappearance, and her reappearance with wild stories of travelling ten years in the future, their relationship had changed. Chrome doesn't need his help, in fact Chrome never needed his help, she had the strength to stand on her own all along, and she just could not access it. Mukuro will admit to himself that he had hoped to be her crutch, to unleash her strength only under his will. Her strength was his strength, and vice versa. But Chrome doesn't need him to power her organs anymore; it's no strain for her to use the Vongola rings and her Mist powers to keep herself running. And she tells him, in that soft, quiet mental voice of hers that she should 'reserve his strength'.
And so Mukuro withdraws, and withdraws, and withdraws. It's a tactical retreat, but a retreat none the less, and he can feel the distant pain of Chrome, Ken, and Chikusa, with each step he takes back. He can even feel the distant, far off echo of Lancia's surprise. Mukuro retreats inside of the tube, his floating prison, and thinks hard about the next step to take. If he admits to himself that Chrome was a faulty plot in the beginning (too sweet a girl, too innocent, and too ready to follow where Sawada Tsunayoshi led) he would have to admit that the urge to topple the mafia and make the streets run red with their blood hasn't been a strong pull since that battle with Sawada.
Sawada did something to him, and for that Sawada should pay, but Mukuro cannot summon up more than a handful of twisted, pathetic intrigues. His life is dark, spent in his own head, until the Vendicare's caring hands turn off the machines keeping him alive, and pull him uncaring out of the warm water, to shiver under cold fluorescent lights.
"Mukuro-san." Chrome's soft hands had touched his face, tilted it so she could see into his eyes, and Mukuro knew, that there was nothing more for him there. That Chrome would be Chrome would be Chrome… and that it was her name now.
Mukuro met Irie after his first week of 'probation'. Sawada did not bother with gates or guards or fences or dogs, he simply assumed Mukuro would behave. Mukuro felt like a whipped dog, unwilling to pass the line that marked where the electric fence was, even though the fence had been turned off. He had been sitting in a favorite café of Reborn's, enjoying a cappuccino and the sight of a flowering sakura tree, he could feel Chrome's worry, and Ken and Chikusa's quiet loyalty. It was a loyalty Mukuro couldn't draw on now, he had tied them to Chrome to tie them to him… and now there was no way of untangling it all without a mess.
"Excuse me, but is this seat taken?" A boy, a teenager, Sawada's age asked, heavy books thrust under one arm, and a small black cup of coffee in the other. Mukuro shrugs, allowing the teenager to take it. He's not an unattractive boy, but he is unobtrusive, quiet in a way generally found in geeks and nerds when in an environment not their own.
"Do you come here often?" Mukuro questions, engaging the teenager in conversation. A small part of him enjoying the confused, innocent expression on his face, like he was unused to being noticed.
Like Chrome had been.
It doesn't take much to learn his age, his school (Namimori, of course), and his home life. It would take much less to scratch him under the table, control the boy, and ruin his quaint little life. But Mukuro is bored with things like that, and he cares little about time travel (in theory, in reality it's fascinating), and even less about robots. So he lets the boy go.
"Mukuro Rokudo, this is Irie Shouichi." Sawada Tsunayoshi introduces them years later, in a different setting, where a Mediterranean wind blows and Mukuro has to create illusions to enjoy sakura trees. Irie is still unobtrusive, nearly forgetful, except his hair is red, curling and spiking away from his skull, and his hands rarely stop moving.
"We've met before." Mukuro says, and Sawada is surprised, but introductions go on. Irie it seems, was an ally of them in the future, and he does not remember that. He does, apparently, remember shooting them with the ten year bazooka, but has no memory of any experiences that the Vongola Guardians have had. Gokudera Hayato is rebellious, and Yamamoto Takeshi uncaring. Hibari Kyouya looks accepting, for once, and this is enough to perk Mukuro's curiosity.
Chrome's quiet acceptance of all things is unstartling.
Mukuro floats on the outer reaches of the Vongola Guardian's circle. He is too dangerous to be left on his own, too powerful to not be included, but too dangerous to be fully included. It's a conundrum, and fascinating to watch the others work through. Ryohei Sasagawa apparently decided that anyone who can get under Hibari's skin like that is alright with him, but Gokudera Hayato eyes him with open distrust, and Yamamoto Takeshi simply smiles.
Hibari, at least, puts his money (and his fists) where his mouth is.
"Hibari-k-" Irie Shouichi cuts himself off when he opens the door to the clang of metal on metal, trident versus tonfa. The combatants freeze, and Irie's eyes flick between the two of them. No doubt he notes that Mukuro is dressed only in a loose bathroom, hanging open from where Hibari ribbed through the tie holding it closed, and that Hibari is wearing a towel barely large enough to cover himself with. "…" Irie closes the door, and is gone, rushing off to closet himself with whatever research he needed to talk to Hibari about. Mukuro smirks at Hibari.
"Oya, oya, you don't think he got the wrong idea, do you, Skylark?"
It's funny, that now that Irie is avoiding him, Mukuro searches him out. In other places, with other people, it would be called stalking. "You know, Irie-kun," Irie jumps when Mukuro tugs his oversized headphones down, "the way you work, someone would think you were searching for something." A diagram of a cube whirls on Shouichi's screen, coming apart and coming together while diagnostic read outs scroll down the sides.
"Rokudo-san!" Irie blinks at him, eyes owl wide, before he focuses. "Well, I am searching for something Mukuro-san, the boxes are a fascinating scientific conundrum. They were created-" And Irie begins to ramble on, about things Mukuro doesn't care about, and doesn't need to know about. Boxes? Boxes he understands. Your ring and power goes in them, a weapon comes out. Irie talks in diagrams and decagrams, things Mukuro doesn't even pretend to understand. He never bothered paying attention in school after all.
"Keep up the good work." Mukuro says, and lets his hand linger.
Shouichi's letting Rokudo Mukuro fuck him because it feels good. It feels good, and there's no reason no to, and no one to stop them. He knows it's wrong, or at least not right, and it makes Sawada uneasy. When Shouichi is with Mukuro he's being used, and he's being used in a way he understands, not in the careful, hidden meanings behind Sawada's words. It has nothing to do with a timeline that no longer exists, or experiences that Shouichi hasn't taken part it.
And when Mukuro transforms himself into a face that Shouichi has only seen in dreams, speaking in a voice that Shouichi hears when the wind howls at night. That's when it all makes sense, the sense of loss, the strange uneasy sense of something unfinished. Shouichi wonders if it's a memory of a past life, someone he never said a proper goodbye to. He doesn't know, and it haunts him.
It's strange that the man the Vongola consider a ghost helps.