Dan Katō

was a jōnin level ninja of Konohagakure.

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 Shinobi World War, and, despite Tsunade's attempts to save him, died of his injuries.

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 pocket-less flak jacket.

Abilities
Not much is known of Dan's abilities but by all indications, he was a very capable and efficient ninja that served his village well. This is furthermore proven since Kabuto thought he was worth being resurrected. Dan is noted to use a jutsu called the "Spirit Transformation Technique". Although it is unknown what this ability entails, Dan suggests it can be stopped by trapping him in a barrier before the technique's activation. In response to this, Chōza Akimichi remarks it isn't easy to put up a barrier when fighting Dan.

Shinobi World War Arc
Dan is resurrected by Kabuto to fight against the Allied Shinobi Forces during the Fourth Shinobi World War. Dan and several other revived shinobi are sent with the White Zetsu Army to the coast of the Land of Lightning, where they engage the opposing First Division. Dan begins fighting Chōza Akimichi, remarking when their battle begins that Chōza has grown since their last encounter. As the conflict continues, Dan asserts that he must be captured before he is allowed to activate his Spirit Transformation Technique.

Legacy
As Dan had died from fatal blood loss, which Tsunade had failed to cease, it caused her to develop hemophobia. This leaves her unable to participate in additional battles and she leaves Konoha, taking Dan's niece, Shizune, with her. Years later, Tsunade is approached by Orochimaru, who offers to bring Dan and her brother back to life. As this would require two living sacrifices, 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 made a cameo appearance in the ending omake of Shippūden episode 129 alongside several other deceased characters.
 * 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.
 * Dan is left handed.
 * Dan's name is likely a reference to historical ninja Katō Danzō.
 * Despite wearing the pocket-less flak jacket when he was alive and when he is first shown resurrected, he is shown fighting Chōza during the Fourth Shinobi World War wearing a modern flak jacket with the pockets on it.