Mito was a kunoichi born in Uzushiogakure, the shinobi village from the Land of Whirpools. Hashirama was the founder of Konohagakure and the first Hokage. Both belonged to very strong clans who were distant relatives, the Uzumaki and the Senju, respectively. At some point, they met and married. Later on, when Hashirama battled against Madara, who was controlling the Kyuubi, Mito was there to assist him. To repress the beast, she sealed the Kyuubi within herself, turning into the first Jinchuuriki of the Nine Tails.
Some time later, they had a child who would later become the parent of Tsunade and Nawaki, their grandchildren.
- It was revealed by Kushina that Mito married the first Hokage, Hashirama.
Among the Fans
Although the pairing is known among the fandom, it's usually overlooked; mostly because the spotlight is taken by other more popular parings involving the characters, for example: HashiMada, MadaMito, etc.; or even other canon pairings like MinaKushi.