I'm too tired to copypaste from tumblr so instead I'm going to explain these like a game show host who is really sick of his career and overcompensates for it by being obnoxious and acting like he's married to game shows
TETHYYYYYYYSSSSS is a leafy sea dragon (quite literally). Albeit she's very self conscious about it and rarely seen in that form, unless with Wyatt.
Julius is HEAAAAD ADVISOR OF THE QUEANDARIAN PALACE, KNOWS ASTRONOMY, MAPMAKING, AND ENJOYS LONG WALKS ON THE BEACH WITH HIS HUSBAND WHO ENJOYS STABBING THE SHIT OUT OF THINGS WITH SWORDS
WYATTT was just a normal teen until his parents built a very strange fuck no I'm not going there he has an unusual sight to be able to see monsters on different planes and is currently dating a dragon (Tethys) danny phantom was fucking awful I will not ever go there
Pi is a literal ghost in the machine, a ghost trapped inside an arcade game. Eddy and Frill trying to get him out only resulted in him taking on an odd form outside of stuff
Frill is a teleporter with a tick that sometimes causes him to teleport at random. He likes bugging Pi by teleporting inside his game and bringing him random junk from Queander