The Puppeteer is a high-ranking very scary Umichooan man.

He's described as a big, girthy man with eye-searing garish clothes. Most of these things seem to be a rare divergence among umichooans, however current Riung has mentioned him possibly setting it like this himself.

According to Future Riung he might have been the person who granted Victor Vituska the ability to take Gifts from other people.

According to Present Riung, he's “someone who can decide our deaths on a whim”.
According to AMANADA, he's a “someone who we can't defeat (yet)”