He was born with the current, the seas and oceans were his body and storms his blood. When he was young, just created and still under the care of the beings that called themselves his parents, he was vicious and ever-changing. He was as prideful as his rivers, believing himself stubborn enough and strong enough to get anything he desired, just as rivers carved their wanted paths through sand and stone. He was competitive, as well, particular with his twin. She was the sun and light and hearth, high above and ever out of reach. He had hated and adored Amaterasu in the same breath, the ocean having claimed her and angry over the idea she was so out of his reach. Their younger sibling was the moon and stars and the dark, able to soothe away their arguments with soft words and softer smiles. And so, the three of them were as inseparable as they were destructive and healing.
Or they were, at least. Every story has its end, however, and theirs started with a simple argument, one among many. They weren't even arguing amongst themselves. They were supporting each other, protecting each other in the face of a newcomer they weren't sure they could trust. Tsukuyomi had been the one to bring Kaguya back to them, a broken woman once a human. They adored Kaguya although it was clear she did not feel the same about Tsukuyomi. They gifted her the moon. He had panicked at the idea of Tsukuyomi weakening, at the idea that they might prefer Kaguya's presence to their own sibling's. He had lashed out first at Kaguya, Amaterasu standing strong by his side, and then at Tsukuyomi, fear driving him. Amaterasu was always the quicker temper of them, always the first to jump to conclusions and to act on them whether they were right or wrong. It tore them apart, though they hadn't let it be easy. They were all each other knew, after all.
Tsukuyomi was the first to leave, following Kaguya out of their lives when the arguing and the tension and the idea of not being safe with their own family got too much for them. Amaterasu and he tried, more afraid of being alone than anything, because if they separated they would truly be alone. They had no 'Kaguya' to follow, to stay beside. Eventually, though, they broke apart, seething with too much rage to try any longer. He likes to think that's why he fell into the trap like he did but truth be told, he'd always looked down on humans and their inconsequential schemes. Yet in the moment, more than anything, he missed his siblings, whom he always thought would be there to fill any of his faults.
He likes to think he learned from his mistakes, that he could return to them when he's healed and free and they wouldn't turn away in disgust. Really, though, he's simply learned to hide his roiling emotions, pretend his anger and glee and sadness don't exist. Bury it all under an apathetic face and never-ending curiosity. He has changed so, so little. It's evident in the way he clings to the children that would be his brothers if he wasn't a trapped god. It's evident in his simmering rage at Butsuma, at Tajima, at all the people who threaten these children. It's evident in the way most of his morals are set aside in favour of his experiments. It's evident in the way the Naka river calls to him, sets him at ease.
It's evident in the way the Uchiha draw him in, painted in Amaterasu's power as they are. He doesn't use his strength against them, too lost in his memories of Amaterasu, not until his inaction nearly loses him his new brothers. Then it is like a switch is flipped and he is straining against his binds, desperate to not lose another family, desperate to protect and love and keep like waves dragging you farther and farther into the ocean's clutches. He is as much a storm on the battlefield as he is allowed, water follows each twitch of his fingers, ancient power thrumming beneath his skin, escaping in horrifically watered-down attacks. He blinds himself with it, the mortal body he's bound to unable to withstand a god's anger. It doesn't stop him, doesn't get him to even pause. Remnants of his sister's power burn him with each fight, scorching him for daring to go against Amaterasu. Still, he clings to Itama and Kawarama and Hashirama, not these strangers that may have once been blessed.
He thinks he was blinded by anger and that's why he didn't realize until his bonds were loose with Butsuma's demise, storms taking up residence in his too-human blood, power crackling around him. The taste of Kaguya's power foreign and familiar on his tongue, burrowed into Hashirama and into Madara like poison. He wants to laugh. Wants to cry. Wants to tear Kaguya apart for daring to puppeteer not one but two of his families. His rage doubles down, weakened bonds creaking. They hold but only barely. His power is clear, his nature is clear. Izuna's fear, Madara's fear, tastes like ash on his tongue.
The war is over. Just like that, the mortals lay down weapons they had been wielding for generations. Kaguya should have never let him fall, let him go free among and grow attached to her playthings. They are his, now. They are his in Hashirama's boundless hope and dramatics, they are his in Itama's solemn curiosity and talent for healing, they are his in Kawarama's trickster nature and in Touka's sharpness, they are his in Izuna's ability to match his human binds. They are his in Madara's awe and admiration, in his growing adoration.
He lets his mind dance among the projects needed for a village, playing his part and never giving too much. He has changed in their eyes but he is the same he has always been. He encourages and teaches and loves. He lets interest and emotions lead his way beneath a bland mask and sarcastic words. He lets himself flit through the village, lets himself collapse and open and talk until there is nothing left to hide and his fingers can press into smooth skin. Admiration and adoration turn to like turns to love and he has a village and children to teach and family to hold close and a man to love. That is how Amaterasu find him. Eventually, it is how Tsukuyomi finds them.
Someday, Madara will grow old. Someday none of the humans he loves will live on the mortal plane. Someday, he will have to turn his attention to Kaguya's schemes and trickery. Some day, Tobirama will have to be Susano'o. Someday is not today, however, and today, he loves and he lives.
