Shizune

is a Jōnin-level kunoichi of Konohagakure. She has served as Tsunade's personal assistant for years. Shizune was also Tsunade's apprentice, becoming a talented medical-nin. Shizune remains quite close to Tsunade, being one of the few people Tsunade would trust with absolute certainty, and a fairly talented and capable ninja in her own right.

Background
Shizune is the niece of Dan, the late lover of Tsunade. Some time after Dan died, Tsunade left Konoha, and Shizune went with her as her attendant. The bond between Tsunade and Shizune is strong and, above all, Shizune sees her purpose in life as caring for Tsunade.

Personality
Unlike Tsunade, she is level-headed, practical, and a rational thinker. As a result, she is the one who tends to stress over details, such as Tsunade's tendency to gamble and to think with her heart. She often gets comically worked up over any irrational decision Tsunade makes. Shizune also looks after Tonton, Tsunade's pet pig.

She's not the type to tire easily and is very energetic while at work, often urging Tsunade to finish the paperwork in order to avoid mountains of unsigned documents later on. She is often seen asking Tsunade questions about what to do with the village and the shinobi on missions.

Medical Ninjutsu
Shizune is noted to be a very talented medical ninja, in Konoha rivaled only by Tsunade herself and Sakura Haruno, another student of Tsunade's. Her training has blessed her with excellent chakra control, which is the primary reason she was assigned to lead the sealing of the Three-Tailed Giant Turtle. She demonstrated her great medical knowledge when she lead a group of medics in healing Neji Hyūga, even though she had only recently returned to the village after many years of wandering the world with Tsunade. This was again seen when she was put in charge of the autopsy Konoha performed on the Animal Path that Jiraiya had killed.

Fighting Skills
Shizune has rarely been seen in battle, but it is apparent that her attacks are based (ironically, due to her medical talents) solely around poison. Being a medical-nin, she has to keep her distance in battle, making sure not to injure herself so she can be of use in healing her teammates later. However, she is quite fast and equally fierce and precise in her attacks, as stated by Kabuto.

During her battle with Kabuto, Shizune is seen using Poison Mist, which creates a thick cloud of toxic gas capable of being fatal even if only a mere whiff is taken, as noted by Kabuto. She also uses poisoned senbon in a variety of ways, including firing them from a wrist mounted device and spitting them out through her mouth. Kakashi Hatake mentioned that Shizune was responsible for instructing Sakura on how to imbue her weapons with poison.

Search for Tsunade arc
When Orochimaru came to Tsunade years later in an attempt to get her to heal his arms, Shizune attempted to persuade Tsunade against it, even trying to convince Tsunade to kill him with her assistance. When all attempts to persuade Tsunade failed, however, Shizune tried to use force to stop Tsunade from helping Orochimaru, although Tsunade easily defeated her. She later fought Kabuto Yakushi in an effort to protect an incapacitated Tsunade and Naruto Uzumaki. Despite Shizune's efforts, Kabuto proved too much for her, as he easily dodged her attacks and incapacitated her, too. Shizune managed to heal her legs, and left the field of battle with the unconscious Naruto as the three Sannin prepared to conclude their battle. She then returned to Konoha with Tsunade after she accepted the office of Hokage.

On the way back, the group stopped in a hot springs town. When Senta and Bunzō had stolen the First Hokage's necklace which was currently worn by Naruto, the two plot to blackmail Tsunade to give them the money she owes their boss. When Shizune found the note, she couldn't find Tsunade anywhere which left her the choice to use the Transformation Technique to pose as Tsunade. When Naruto and the Akagi brothers stole the bag, Shizune in Tsunade's form goes after them. After Shizune took down Naruto, the Akagi brothers discovered that there's no money in it. Just then, the real Tsunade arrived and recognized Shizune in disguise. With help from Jirōchō Wasabi (whom Tsunade encountered in the casino), Tsunade explained that she already paid her debt to the Akagi the year before.

Sasuke Retrieval arc
When Tsunade took on the title of Hokage, Shizune (in her Jōnin garb) led a team consisting of Genma Shiranui, Raido Namiashi, and Iwashi Tatami to compensate for the lack of manpower caused by the Invasion of Konoha. When Genma and Raido were defeated by the Sound Four, Shizune healed their wounds. Later, she was seen leading a medical team charged with regenerating the wounds of Neji Hyūga.

Anime Filler arcs
She appeared in almost every filler arc as Tsunade's attendant and secretary. During the Mizuki arc, she was defeated by Mizuki, who used her guise to lead Naruto and Iruka into a trap.

She was with Tsunade when going over the Land of Vegetables crisis. When the uprising in the Land of Vegetables and the caravan that Naruto Uzumaki, Chōji Akimichi, and Hinata Hyūga were guarding was connected, Tsunade sent Shizune to gather every available ninja and head over to the Land of Vegetables immediately. While Naruto was fighting Renga, Shizune, Shikamaru Nara, and the ninjas sent with her rescued Chōji, Hinata, and Yurinojō from the ninjas on Renga's side.

Part II
Shizune appears frequently in Part II, often being nearby when Tsunade is issuing orders.

Sasuke and Sai arc
She began to grow concerned by Naruto repeatedly going up against members of Akatsuki, and reported this to the village elders. Although Tsunade was able to reach a compromise with the elders, Shizune doubted Tsunade's wisdom, but she seemed to regain faith later on.

Three-Tailed Beast arc
In the anime, Shizune was assigned to help seal the Three-Tailed Beast, joining the sealing team along with Ino, Sakura, and Hinata. Seeing the beast's sealing as a way to stop Akatsuki's plans, Shizune took charge of the process, instructing the other members in order to speed along the process. The sealing was repeatedly interrupted by Guren and her men, however, and the sealing was instead left to ANBU members.

Six-Tailed Beast arc
In the anime, Team Kakashi while under Yamato's command, is sent to aid the near-extinct Tsuchigumo clan in guarding its dangerous forbidden jutsu from enemies. Later while Shizune is doing her usual duties as Tsunade's assistant, Team Kakashi sends word to Tsunade about plans to destroy the forbidden jutsu with the clan's consent. Shizune wonders what Tsunade will do.

Invasion of Pain arc
After Jiraiya was killed by the Akatsuki leader, Pain, Shizune was left in charge of examining the body he had captured. When Shizune discovered that the piercings on the body were chakra receivers, she was given ANBU escorts to ensure she could share the information with as many people as possible. She meets up with Inoichi, but before they could begin to collaborate, Shizune was captured by the Human Path. It read her mind to find out that Naruto was at Myōbokuzan, and then removed her soul, killing her. In the ensuing destruction, her body was kept safe, and was later revived by Pain's resurrection jutsu.

Five Kage Summit arc
Shizune was later seen with Naruto, Sakura, and Kakashi, looking down on Tsunade, who was now in a coma. She was later seen explaining to Samui that Tsunade was unable to take the letter meant for the Hokage, and was shocked when Danzō arrived and stated that he was the new Acting Hokage. She is later seen tending to Tsunade and is also present when Sai tells Sakura how Naruto feels for her.

Fourth Shinobi World War arc
Days later, Shizune senses a heartbeat from Tsunade. She sends one of the shinobi standing guard over them to announce Tsunade's awakening to the Konoha Council and the Fire Daimyo, who was about to appoint Kakashi Hatake as the Sixth Hokage. Shizune is then seen throwing herself on top of her recuperating master, crying out in joy and "squeezing too hard". Shizune was later seen at Tsunade's side when Tsunade was eating a lot of food to regain her chakra. She also panicked when it looked like Tsunade considered eating Tonton.

Other Appearances
Shizune is commonly a playable character in the Ultimate Ninja series of games. She is seen to be able to create swords out of chakra in a move called "Chakra Sword: Great Cross Slash", which causes both physical and internal damage to her opponent, and creates a huge explosion. She also has a jutsu called "Hog Style: Tonton", which calls on Tonton to attack Shizune's opponent, hitting the opponent up to 30 times to cause major damage.

Trivia

 * She is one of the few named characters in Naruto to experience death and be brought back to life. The others to have experienced this are Gaara, Kakashi Hatake, and Fukasaku. A lot of other Konoha ninja shared this fate, but their names are unknown.
 * It was shown in a post-credits omake that despite her devotion to Tsunade, she is also wary and insecure about their being single way past the perceived marriageable age, even expressing jealousy to Kurenai Yuhi's perceived relationship with Asuma Sarutobi. Though this was blurted out in a drunken stupor, it did not stop Tsunade from chastising here.

Quotes

 * (To Naruto) "You may think you're all healed, you may even look all healed, but you're NOT all healed!"
 * (To Naruto) "All in good time, just as soon as you're completely healed."