Dan Katō

was Shizune's uncle 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.

Appearance
Dan had long pale blue hair that extended into his back and dark green eyes. He also wore the standard Konoha shinobi outfit of that time complete with forehead protector and the pocketless flak jacket.

Abilities
Not much is known of Dan's abilities but by all indications Dan was a very capable and efficient ninja that served his village well. This is furthermore proven since Kabuto thought he was worth being resurrected.

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.

Shinobi World War Arc
Dan is resurrected by Kabuto, along with other well noted shinobi through the Summoning: Impure World Resurrection, to prepare for war against the Allied Shinobi Forces.

During the advance of Akatsuki's frontline Dan talks to Asuma Sarutobi and Hizashi Hyūga, explaining the reason behind their resurrection. After reaching the coast of the Land of Lightning, Dan, the other revived shinobi, and the White Zetsu Army enter in to battle with the First Division. He confronts Chōza and comments on how big he has grown.

Trivia

 * Dan, along with other characters from the Naruto saga, made a cameo appearance in the ending omake Shippūden Episode 129 to where he was seen standing next to Yashamaru.
 * According to the Naruto databook:
 * Dan had completed 574 official missions in total: 77 D-rank, 84 C-rank, 223 B-rank, 168 A-rank, 22 S-rank.
 * Dan's favourite word was.