Kosetsu Ryu was Shirahime’s publicly known zanpakuto during her time before becoming fifth division taicho. His name means “Snowfall Dragon” and when used with the physical sword form, he is a white snow dragon.
He often appears in physical form without the sword as a small ice dragon who likes to play with Yachiru. He is protective of Shirahime and her children.
Kosetsu also appeared as a boy with snow dragon wings in Fifth Division in the Red’s Chapter 7: A Prize. Here Shirahime calls him her son and Daichi calls him his and Yachiru’s older brother.