Note : This article may ruin your childhood
Pop n Music was a part of a scientific project and a segment of [popnmusic.wikia.com/wiki/MZD MZD]'s imagination. [popnmusic.wikia.com/wiki/Yuli Yuli] was a friend of MZD who passed away while he was a child. So MZD wanted to recreate/reincarnate his friend in an outer world so that he could meet him again. He does remember what Yuli looks like, so he pictured him as a vampire with blue hair.
Yuli is a person who likes music more than anything else and wanted to be a musician before he died so to satisfy Yuli, MZD created a world where the inhabitants need to play music in order to survive. That's why Yuli and his band are the only ones involved in music while the others have different personalities. MZD thought Yuli would be alone inside the world of music, so he created Ash, Yuli's dog and Smile, a kid who passed away the same day. MZD wanted a mascot, so he used an anthropomorphic rabbit and cat as the mascot of his program.
The last night he woke up, he found out he was in his own world. He soon became known as the mastermind of the project, so he used his subordinates to challenge and control the player.
The real Yuli doesn't like the idea of this, so he woke up and decided to stop MZD using programs as his [MZD] subordinates. He later knew about MZD's project, so he went to the world of Pop'n and team up with Ash, Smile, Poet, Nyami, Mimi and the other characters to stop him.