The Clock
 

This Imaginary Friend only appears in the episode Goodbye to Bloo. Everytime an hour has passed, he says "(no.)o'clock to alls well!") Bloo keeps getting annoyed by him, and tells him to shut up more than once.

My favourite scene is when he is just about to answer the door but Bloo throws him out of the way 8D

The artist who painted the portrait of Mac and Coco and also, took the last sighning space on the card also only appears in this episode too.

I laughed so hard at that scene.