Hisagomaru is the manifested spirit of Hanatarō Yamada’s Zanpakutō who is seen during the anime-exclusive Tōjū Campaign.

Hisagomaru is a simple toy robot about as tall as a three-year-old child. He has a cylindrical body with two arms and two legs and flat plates for hands and feet. Hisagomaru’s head is perfectly round, with one normal eye and the other replaced with a gauge which will fill as he heals people, much like the one on his sword form. His chest can open up to reveal an enormous cannon.

Much like his partner, Hisagomaru is very timid and insecure. Unlike the other Zanpakutō spirits manifested by Muramasa, Hisagomaru has no personal desires or problems against Hanatarō. In fact, when Muramasa forced him to materialize, he continually attempted to return to his sword form, going into great despair (and into hiding) after thinking Hanatarō had abandoned him.

(Source: Bleach Wiki)