In case one day I don't return
In case one day I never see you again.
Tobirama refuses to be the romantic type, and he's made it clear to Mito since before they were married. He has a reputation, and he wants to take care of it. However, over time—a long, long time—Tobirama starts giving his wife a flower a week.
A simple one, without any embellishments or foliage. A freshly cut flower that smells of grass and spontaneity.
Mito knows Tobirama likes fuinjutsu, so that's their way of spending time together. In the study, with books, scrolls, and ink. That's what characterizes them.
He wants peace for his village.
She also wants peace for the world ninja.
xxxxx
Tobirama always knew that the shadow of death follows ninjas wherever they go. Now he's no longer a child: he's a jōnin of Konoha and has his own team of genin under his command. He knows he has a responsibility to his village and the children in his care, and while the skilled Senju doesn't show fear in front of everyone, his heart flutters with every mission assigned to him.
It is then that he kisses Mito.
“In case I don’t come back one day,” he says.
Although missions with children don't pose any great danger, he always kisses her and repeats the same words to her. With experience, he does it as a joke, and Mito laughs, assuring him that when he returns, the food will be ready for him.
He also carries out other types of missions. No, Tobirama Senju isn't just any ninja: he's part of the elite. As a jōnin, he often teams up with Izuna Uchiha.
On this mission, Tobirama is wounded, and Izuna dies. He sincerely believes he will keep his word and that the kiss he gave Mito will truly be his last, but his will prevails, and he resists. This time, he returns home with tears in his eyes and showers Mito with kisses, while hugging him tightly and gently. Mito soon discovers the reasons for his effusiveness.
xxxxx
When she becomes Hokage after the death of her brother, Mito is proud of her husband and her family, her children, along with her little granddaughter, Tsunade.
Now he's the Second Hokage, it's his duty to protect the village. His missions are reduced, but when he goes with the intention of alliances or SS-rank missions, it's always the same kiss: a short, chaste one, on the tip of his lips.
“In case I don’t come back one day,” he says, disappearing into the darkness.
Xxxxx
When the Second Ninja War breaks out, Tobirama had kissed his grandson Nawaki before going on his first mission, where he died. After that event, he can't sit still, so he teaches Mito all the clan techniques he's learned from his family for generations. Mito is an excellent student and a devoted wife. She also teaches Tsunade, who is more practical and outgoing than her grandmother.
One night he gives two kisses, but neither is goodbye.
One is given to her by Tsunade, naming her heir to the Senju clan along with its secrets.
Another one is given to him by Mito when he says goodbye at night.
Neither of them thinks it will be the last. Not when their granddaughter has just become the heir.
But dreams fade like bubbles.
A ninja reports an emergency, which Tobirama attends with his former genin team, now jonin. Everything goes wrong, and Tobirama names Hiruzen the next Sandaime, in addition to giving him a special scroll for Mito. He then turns and confronts the Iwa ninja.
And the last thing Tobirama thinks before dying, after defeating the Silver and Gold brothers, is that he said goodbye to Mito so many times in the past, and this time it is no longer necessary.
He has gone ahead without knowing it.