Even in his old age he remained healthy, because the glory of his father had been how even in his sixties he had stood taller and firmer than any other man, and yet beyond basic health it was obvious his skills were declining, his strength was fading. He'd tried to fight the current of time, dyed the greys vainly, spent two weeks in a foul temper waiting for his lungs to clean and wishing Otacon would stop hugging himself over the fact he was deliberately making it look like his idea. The medical tracking nanoshots revealed the scars on his bones where the muscles had pulled against them when they'd been there; Otacon had remarked on that, and the fact that even on the inside he was growing lines of wear and tear like it was some great irony. In a way, the pain of his own body decaying around him was satisfying, cathartic. It was more painful not being able to smoke any more. With his intellect, he still found holes in the arguments, tore stories, spread the truth to the waiting people, trying to build some meme into the culture that would mean They would lose their favourite foothold. He didn't need his body, and like any unused thing, it decayed.
Then Otacon had shown him the suit he had spent so long slaving away at, covered in ridges like the ones around his eyes and mouth, armoured and guarded yet organic and beautiful. He jammed the initial nanoshot into his arm stoically, and placed the suit on piece by piece, smoothing it over his weakening, naked body, until fully dressed. Otacon looked at him, proudly, and opened his mouth as if he was going to speak, and then didn't - simply tapped something on the PDA, and his head buzzed with the nanos all standing to attention, and he felt strong. The suit had been modelled after the sleek, streamlined form he'd had in his thirties - just like the Metal Gears, the ideal soldier had been getting smaller and quicker and more agile, no longer hulking tanks but fluid and highly perceptive, and that was the plan he had been built to - it was pure, intelligent efficiency. But it was nicer to think it was because that was the way he'd looked when he'd first met Otacon, he decided, sinking into the nostalgia of a life overlived.
He moved his arm experimentally. Muscles rolled like mercury down a test tube wall. He turned the movement into a punch, raising his other arm smoothly, attacking the air with the viciousness of a powerful mind in a decaying old body, chaining the spinning kick into the kind of effortless flip he couldn't see any more without thinking of the kid and laughing, and closed his eyes, letting the nanomachines scrape away the mist, give him the illusion of youth until the batteries died.
He did that for a while; move and sway and fight and perform all kinds of stupid acrobatic stunts that would get you shot in the real world all for the sheer hell of being able to do it. He was a dead body in a cyborg exoskeleton, and while he loathed the resemblance, he felt like screaming in the joy of being in an awake, alive body, as sharp as his mind still was; not feeling the weight of his own bones or the grinding in his joints. Otacon talked him through the features, and he hardly listened - if he ever used this thing Otacon would no doubt repeat it to him like he was a kid playing a messed-up video game and didn't know what the buttons did. The fire in his mind was too intense to fully listen, anyway.
Eventually Otacon asked him how it felt, and he dodged the question by pointing out that Otacon had used the wrong kanji on the logo and had written 'dark blue' instead of 'soul', and Otacon had flailed around for a short while with his Japanese dictionary before ordering him to show him how to write out the real character. He hadn't written any Japanese since he was in his twenties, and was slightly amazed at the fact that his hand still wrote the strokes down in the right order without him having to think about it at all. He wondered how long it would be before he could no longer do that.