Snowball is the assistant drone of Mei. The drone is usually seen stored on Mei's ice tank of her back and is resposible for her ultimate, Blizzard. It can also be seen accompanying Mei in her emotes and highlight intros.
In Mei's Adventures, Snowball is described as "not the greatest conversationalist." He does, however, appear to have at least some explicit personality; in the 'Boo!' highlight intro, he is shown specifically laughing when he spooks Mei enough to cause her to use her ice block power.