A slime Mukouda befriended as his familiar due to its general cuteness and usefulness when cleaning his garbage from another world. It usually hangs inside of Mukouda's bag, as a travel companion to him and Fel.

Thought to be a regular water slime at first, Sui, thanks to Mukouda's food providing ridiculous stat boosts, quickly learns how to use magic, how to speak, and how to produce high grade medicine.

Sui is shown as being highly excitable and very supportive, extruding a positive attitude towards life, be it helping out its companions or killing powerful monsters.

When Sui likes something, it creates a heart-shaped emote inside of it's body.