He meets her when he is seventeen and she is fifteen. They slink out into the night, they dance and fuck and fight, and in the end, go their own ways.
He does not know until several years later, when his brother and Madara have founded Konoha, but dthat one night he spent fucking her gave him two children. When he finds out about Tajima and Akusa, he is angry and hurt, but realistic enough to realize any communication could have killed them both. What soothes the sting is the way his son quickly goes from calling him
"Otou-sama" to
"Tousan" and his daughter begs to meet his snow leopards. Aya herself looks at him with guarded hope even though he cut down Izuna. He asks her why, of course.
"Oh Tobirama," He appreciates that she doesn't insist on using
"Senju-sama" for he detests the title.
"You gave me two children. Who are, aside from enormous cunning and unending loyalty, entirely Senju. Despite them being born bastards, hidden from you and your clan for safety, you have accepted that they truly are your children. Izuna-" Here, she turns away to study the skyline
"Was my brother, but he was also a shinobi." She turns to gaze at him again.
"He loved me, and wanted me to be happy, Tobirama. I had Tajima and Akusa for a reason, and I believe that reason was to draw me closer to you."
He has to kiss her for that. She's a gorgeous Uchiha of the main branch, has a terrifying Sharingan of her own, yet she is drawn to him, Hashirama Senju's albino younger brother, the necromancer.
She spends months working with select Kenjutsu students, training their children and handpicked combat partners in the Uchiha Iron Steel style. When Tobirama asks her to marry him, she says yes without hesitation, and Akusa and Tajima whoop and dance like the five year olds they once were.
Their wedding is traditionally Uchiha and Senju, where a not pleased but still cordial Madara gives away his final sibling, and Hashirama officiates his first wedding as Hokage. Tajima insinuates that if Inosha is patient, there is a similar event in her future. Aya beams at her son, but has to restrain Tobirama when Saru finally asks to court their Akusa, who doesn't even give her parents the chance to answer, simply says "Yes!"
The day she dies is the day his world loses all but the rarest wisps of colour. He wants to blame his daughter for letting it happen, but Aya tells him
"I needed to give you this, Tobirama, not as my husband, but as my Hokage." It shatters him again to the point that he wails his heartbreak to the sky. His children are pulled over to their dying mother by their combat partners, and Aya gives them each a final word, mostly for the children, but somewhat for each pair. Her very last words are to tell him she loves him.
His relationship with his children only ever improves, even after the death of their mother, his wife of few years, beloved of many more. Tajima courts Inosha Yamanaka, a girl who is an anomaly among her people- an extra, she has no Akimichi or Nara to back her up. Aya had been impressed by her command of poisons, and had ordered her to come to the Hokage's residence for training in her family's Iron style. Tajima had been delighted to finally have an Iron Daughter to complement him as a Steel son.
Akusa, of course, was dancing around Hiruzen Sarutobi's and her mutual interest the moment they met. Once in a while, Tobirama thought Homura and Koharu might be jealous or disappointed, but they never show it. Akusa changes her name and gives birth to a girl she named in her mother's honor as she had been directed in Aya's will, all before she is sixteen. Through it all, Saru stands where she needs him. And it pleases Tobirama that his daughter, beloved to him nearly as much as her mother, will be cared for- by his star student, no less!