Shizune

Shizune (シズネ) is the assistant of Tsunade and a talented Medic-nin. Shizune is quite close to Tsunade, one of the few people Tsunade will trust with absolute certainty and a fairly talented and capable ninja in her own right, often using jutsu involving poison (such a Poison Mist and Prepared Needle Shot).

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 like Tsunade's tendency to gamble and 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.

History
After her uncle Dan died, Shizune left Konoha with Tsunade, as her apprentice. When Orochimaru comes to Tsunade years later in an attempt to get her to heal his arms, Shizune attempts to persuade Tsunade against it, even trying to convince Tsunade to help her kill Orochimaru. When all attempts to persuade Tsunade fail, Shizune tries to use force to stop Tsunade from helping Orochimaru, though Tsunade easily defeats her. She later fights with Kabuto Yakushi in an effort to protect an incapacitated Tsunade and Naruto Uzumaki, but Kabuto proves too much for her as he easily dodges her attacks and incapacitates her as well.

When Tsunade takes on the title of Hokage, Shizune leads 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 are defeated by the Sound Four, Shizune heals their wounds. Later, she is seen leading a medical team charged with regenerating the wounds of Neji Hyuga.

Sai and Sasuke Arc
In Part II, Shizune expresses concern in Tsunade's decision to allow Naruto to go out on missions, and reports this to the village elders. Although Tsunade is able to reach a compromise with the elders, Shizune doubts Tsunade's wisdom, though she does seem to regain faith later on.