Dan Katō

was a renowned jōnin from Konohagakure.

Background
After the death of his younger sister, he began advocating for medical ninja to be assigned to every squad. Tsunade shared a similar belief, and the two soon began dating because of it. During their time together Dan shared his dream of becoming Hokage with Tsunade — wanting the title so that he could better protect the villagers of Konoha which would prompt Tsunade to give Dan her grandfather's necklace as good luck and encouragement to follow that dream.

Soon after that, Dan was fighting in the Second Shinobi World War, in which he killed countless shinobi with his Spirit Transformation Technique. Ultimately, he was fatally wounded during a battle, and while Tsunade was able to stop the bleeding, it was a lost cause as the blood loss was too severe and Dan succumbed to his injuries. Tsunade would later take on Dan's niece, Shizune, as an apprentice and leave the village some time after that, retiring from life as a shinobi.

Personality
Dan was a great man that deeply loved his village and its inhabitants. He also cared deeply for his sister and would often pick her up from the Academy; something he would later do with his niece as well. He is also described as a wise and honourable man that truly believed in justice. Dan was also very knowledgeable of the history of the the shinobi world as he was very wary upon learning Madara had been reincarnated. Despite learning the five Kage were combating Madara, he was still sceptical they actually stood a chance against Madara and only hoped that the Impure World Reincarnation could be stopped before Madara had a chance to kill them.

Appearance
Dan had long, pale-blue hair that reached his back and dark green eyes. He wore the standard Konoha shinobi outfit of that time complete with forehead protector and the pocket-less flak jacket. After being reincarnated by Kabuto, Dan obtained black sclerae.

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 proven further when Kabuto Yakushi reincarnated him thinking that he'd be a valuable asset on the battlefield. Kakuzu referred to Dan and Asuma as gold and silver generals (a reference to shōgi pieces), again indicating that Dan was a powerful shinobi.

Chakra and Physical Prowess
Dan displayed high levels of chakra control as he could transfer it at will, to which he showed very high reserves as he restored Tsunade's own reserves along with her Strength of a Hundred Seal before departing to the afterlife. In the anime, Dan demonstrated high-level taijutsu prowess along with great speed and evasive skills, being able to single-handedly disarm and defeat several enemies.

Ninjutsu
Dan possessed a unique technique called the Spirit Transformation Technique. This ability allowed him to become a living spirit to attack his opponents. He can also use it to take control of a target's body. Dan also noted that this technique took many lives during his lifetime, and implored his opponents to trap him in a barrier before he could activate the technique. In response to this, however, Chōza remarked it wasn't easy to trap someone like him. In terms of nature transformation, Dan was proficient in Wind, Fire, Earth, Yin and Yang Release.

Fourth Shinobi World War: Confrontation
In preparation for the Fourth Shinobi World War, Dan is reincarnated by Kabuto Yakushi to fight against the Allied Shinobi Forces, before being mobilised alongside Asuma Sarutobi and Hizashi Hyūga, where he informed them of the details regarding the forbidden technique used to reincarnate them. Relocating to the Land of Lightning's coast, Dan emerged from the sea amongst the White Zetsu Army with the others, before engaging the opposing First Division. Confronted by Chōza Akimichi, Dan remarked that he had grown since their last encounter.

As the conflict continued, Dan, in explaining his abilities, tells them that he must be captured before he can activate his Spirit Transformation Technique. After a prolonged battle, Dan congratulated the shinobi on their efforts, while trapped inside the barrier. However, in the wake of the Demonic Statue of the Outer Path's attack, the barrier surrounding him was destroyed, freeing Dan but injuring him in the process.

Resealed within the barrier by the following day, Dan observed the arrival of Naruto Uzumaki's shadow clone to the battlefield, initially mistaking him for Nawaki due to their resemblance to one another. Shocked at hearing that Tsunade had placed her trust in Naruto and his promise to not die until he became Hokage, Dan asked just who Naruto was, to which Shikamaru Nara replied that he was the Nine-Tailed Demon Fox's jinchūriki and had always dreamt of becoming Hokage. Dan smiled as he realised that both his and Nawaki's dream had lived on through Naruto.

Fourth Shinobi World War: Climax
Continuing to converse with Chōza as he guarded the barrier, Dan was greatly shocked to hear of Madara Uchiha's reincarnation, but is even more so by Tsunade becoming Hokage and as such, her entering the fray alongside the other Kage. Despite Chōza's confidence, Dan was adamant that Madara shouldn't be underestimated, believing that the only person who could possibly defeat him was already dead. He then implored Chōza to instead seek out and stop the person who reincarnated Madara, judging this to be the only method to beat him, before silently pleading for Tsunade to survive.

With the Impure World Reincarnation released, Dan is enveloped in a light of sorts and, as he began to deconstruct, commended whomever had stopped the technique. Now ungoverned, Dan requested that Chōza have them dispel the barrier which sealed him before using his Spirit Transformation Technique to take control of his ascending spirit. Thanking Chōza for understanding his intent, Dan left and reached Tsunade in time to take over her body and save her from one of Madara's attacks. Conversing in her subconscious, he thanked her for accomplishing what he didn't in life and apologised for causing her only hardship. He then told Tsunade that he would wait for her in the afterlife, but that she wasn't to follow him yet as she was his dream itself. Kissing her forehead before he disappeared, Dan expressed gratitude that he was reincarnated, because it allowed him to finally protect her. As his final act, he seemingly imparted the remainder of his chakra to Tsunade, restoring her Strength of a Hundred Seal and then his soul returned to the afterlife.

Legacy
As Dan had died from fatal blood loss, which Tsunade had failed to cease, it caused her to develop hæmophobia. This left her unable to participate in additional battles and she left Konoha, taking Dan's niece, Shizune, along with her. Years later, Tsunade was approached by Orochimaru, who offered 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.

Tsunade has never ceased loving Dan, even decades after his death, as she continued to reject her long-time comrade Jiraiya's advances, and never involving herself with any romantic relationships with another man.

Trivia

 * Like Danzō, Dan's name is likely a reference to historical ninja Katō Danzō, a noted ninja master during the time of the Sengoku period of Japan. His family name means "increasing wisteria".
 * Dan made a cameo appearance in the ending omake of Shippūden episode 129 alongside several other deceased characters.
 * According to the databook(s):
 * 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.
 * In the fourth databook, Hiden is listed among Dan's special characteristics, though none of his techniques are known to be classified as such.
 * Dan's ability to cause his soul to exit his body makes him reminiscent of an, from Japanese folklore. An ikiryō is a person whose soul can dwell outside his or her body.
 * Despite wearing the pocket-less flak jacket when he was alive and when he is first shown reincarnated, he is shown fighting Chōza during the Fourth Shinobi World War wearing the old-style flak jacket, which was later changed to a modern flak jacket with the pockets on it in the manga. This was, however, not changed in the anime.

Quotes

 * (To Tsunade)
 * (To Tsunade) "For the sake of everyone's safety... Being Hokage is my dream..."
 * (Last words) "I...can't...die...here..."
 * (To Tsunade) "Thank you, Tsunade… You accomplished more than enough of the things I wanted to do. …I'm sorry I make you feel nothing but painful emotions. I will wait for you on the other side. But please take your time in going there. Because you are my dreams in the flesh. I was able to protect you at the very end… Maybe being brought back by Impure World Reincarnation was worth it…"