An established-relationship MadaTobi with an amnesia trope
I think Tobirama being the one who loses his memories makes the most sense. Because no matter how careful, calculated, and controlled he is, Tobirama is still a scientist who pushes boundaries. He is the kind of person who would risk hurting himself if it meant reaching an answer. He would test things on himself, ignore his own limits, and convince himself that the potential outcome is worth the danger.
And perhaps that is exactly why he becomes the victim of his own creation.
During one of his experiments, something goes terribly wrong. Tobirama believes he has calculated every possible outcome, but there is one thing he failed to account for: the unpredictable effect of his own jutsu. An explosion occurs. Instead of killing him, the technique affects his mind directly, shattering his memories and leaving them broken into countless fragments.
When Tobirama wakes up, he is still Tobirama. But at the same time, he isn't. His memories are not completely gone. They are scattered, disconnected, and mixed together. Some things are painfully clear, some things have disappeared completely, and some memories feel like they belong to someone else's life.
The worst part is that he cannot understand how much time has passed. When he opens his eyes, he is still young. He is still living in the Warring States era. His brothers are still alive.
He searches for Itama. He asks for Kawarama. And he cannot understand why everyone looks at him with such grief.
"Where are they?"
Nobody knows how to answer him. Because to Tobirama, they are still the brothers he lost only moments ago. But to everyone else, they are people who died years ago.
Sometimes he suddenly stands up and says he has a mission to complete. He starts preparing himself as if he was just assigned a task moments ago. As if the war is still happening. As if survival is still the only thing that matters.
Sometimes he falls even further back. To the time when his relationship with Hashirama was falling apart. To the time when Butsuma was still alive. He searches for his father. And the silence that follows is something he cannot understand.
Because Tobirama is a powerful sensor. He has always been able to feel the presence of others. He knows the difference between someone alive and someone gone. He knows the chakra signatures of the people around him. But now there is nothing. The absence terrifies him.
"Why can't I sense them?"
Nobody wants to tell him the truth. Nobody wants to be the person who tells Tobirama that the people he is searching for have been dead for years.
Because he is already broken enough. But the strangest part is Madara. Because the memories involving Madara are still there. They just have no meaning anymore.
Sometimes, when Tobirama closes his eyes, images come rushing back. But they are not memories of battles. Not memories of hatred. Not memories of trying to kill each other. They are memories of something impossible.
Madara waking up beside him in the morning. Tobirama lazily leaning down and kissing him awake. The two of them having picnics somewhere far away from everyone else. Quiet days spent fishing together. The day they finally announced their relationship.
The first time they held hands. The first time they admitted that they had chosen each other. Their first mission together. The first time they moved into the same house. The first night they fell asleep beside each other without a single weapon between them. The first time Tobirama realized he actually felt safe.
So many little moments. But none of them feel real. Because Tobirama has the memories. But he does not have the emotions attached to them. He remembers holding Madara's hand, but he cannot remember the warmth of it. He knows Madara loved him, but he cannot remember what it felt like to be loved by him.
He sees himself smiling, laughing, touching Madara's face with affection. But he cannot recognize that person as himself. It feels like someone has implanted a fake life inside his mind. Like a genjutsu.
And perhaps the most painful part is that Tobirama remembers Madara. But he does not remember loving him. For Madara, it is even worse. Because the person standing in front of him is still Tobirama. The same sharp eyes. The same voice. The same stubbornness. The same brilliant mind.
But when Tobirama looks at him, Madara sees someone who does not know him anymore. He can show him proof. He can tell him their history. He can take him back to their old home. He can tell him every single story they shared. But nothing can force Tobirama's heart to recognize something his mind has forgotten.
And eventually, Madara has to accept something painful: He cannot make Tobirama remember loving him. He can only give him a reason to fall in love with him again.
Honestly, my original idea was something like this:
Hashirama: Tobi, you forgot everything. Madara: You forgot me. How could you? Tobirama: Bitch, who even are you?