Dan Katō

was Shizune's uncle (the younger brother of one of Shizune's parents) and Tsunade's lover.

Background
After the death of his younger sister, he began advocating for medical ninja to be assigned to every ninja squad. Tsunade shared a similar belief, and the two soon began dating because of it. During their time together Dan shared with Tsunade his dream of becoming Hokage, wanting the title so that he could better protect the villagers of Konoha. Tsunade gave Dan her grandfather's necklace as a good luck amulet to follow that dream. Soon after that, Dan was fatally wounded during the Second Great Shinobi World War, and, despite Tsunade's attempts to save him, died of his injuries. As Dan had died from fatal blood loss, which Tsunade had failed to cease, it caused her to suffer hemophobia for quite some time.

Abilities
By all indications Dan was a very capable and efficient ninja that served his village well.

Search for Tsunade arc
After the failed Invasion of Konoha, Orochimaru came to offer to bring Dan, along with Tsunade's brother Nawaki, back to life. This would require live sacrifices, so Tsunade ultimately refused, thinking that Dan and Nawaki would be very unhappy that Tsunade would give up two other lives in exchange for theirs.

Trivia

 * Dan, along with other characters from the Naruto saga, made a cameo appearance in the ending omake Shippūden Episode 129.