A member of the 13 Court Guard Squads. She is a Soul Reaper.
Although she's never directly called "Christmas Cake" in the anime, Rangiku Matsumoto's age is a very sore point for her, with one filler episode going so far as to even create a male version for the sole purpose of having one kid press two berserk buttons by referring to Rangiku as "oba-san" and Yumichika as "o-san" in the same conversation. The anime later takes Rangiku's age references even further by making it such a big issue in the Zanpakutou Arc that it's actually the reason why Rangiku's zanpakutou, Haineko, is angry with her. Averted in the manga, where it's never an issue for either Rangiku or Yumichika, making it an anime-only trope.