Joo-Joo is an Infurnus Slug introduced in the season two, who belongs to Junjie the Eastern Champion and together they protect The Eastern Caverns.

Joo-Joo hails from the distant Eastern Caverns, an isolated region completely unknown to the 99 caverns where Burpy resides. Like Burpy, Joo-Joo is a faithful companion and an ultimate fire-based weapon; extremely loyal to Junjie and his ancestors, being passed down from one protector to the next making him one of the most experienced slugs around. For centuries, he and Junjie's ancestors had used the ancient slugslinging art of Slug Fu to keep the Eastern Caverns safe. But one day, a threat came that even Slug Fu could not stop: an evil slug known as the Goon allied with the sinister Emperor. Junjie's battle with these evil forces led them to the 99 caverns, where they joined the Shane Gang until they could return home to the Eastern Caverns and end the dark reign of the Emperor!

Trivia:
Like Burpy, Joo-Joo is a lone Infurnus wielded by a realm's protector.
He seems to have a similar personality to Burpy, being the "leader" of his slinger's team of slugs.
He and the rest of Junjie's slugs seem to know Slug Fu.
Joo-Joo is the Infurnus of the Eastern Realms.
Joo-Joo seems to be brighter coloured than Burpy, probably for viewers to identify which one is Burpy and which one is Joo-Joo.
Joo-Joo has a little flame shaped mark on top of his head, instead of the lightning bolt mark on Burpy's head. This is likely for viewers to identify which one is Burpy and which one is Joo-Joo.
In Ghoul from Beyond, Joo-Joo's Darkfurnus appearances looks the same as Burpy's; whether than having a "fireworks" kind of flame on his belly just like his normal mega morph protoform, he has the same flame as Burpy.
Joo-Joo was the first one to actually megamorph over 2000 years ago, not Burpy.
Joo-Joo is a boy but he can sometimes be mistaken for a girl.