Penguin is a robot created by Hotaru and has compartments to carry items. It is able to change its shape by rolling into a ball or flattening its body. Originally created for a client, the client returned Penguin back to Hotaru the day Mikan arrived due to it missing her so much. Hotaru said that returned items were useless.

Spoiler

In chapter 43, Penguin proved its worth when it risked its life to obtain Hotaru's cure, sacrificing himself to let the group move on with the mission.

(Source: Wikipedia)