Does Riddim and Yoo Mi exist IRL? In chapter 24.5, the author talks about this one band called Riddim and the lead vocalist Yoo Mi. I tried a google search and the jamaican pronunciation of rhythm was what I got. Here is the where they appeared in the comic.
Riddim is a band in Korea with Yoo Mi as the lead singer. They don't seem to be very popular as Google hardly returns any English results for them. And after doing a quick search of Korea's Billboard top 100, I don't see them anywhere meaning they aren't the mainstream, radio type of band.

Riddim is the band that performs Orange Marmalade's OST. If you read Orange Marmalade from the Naver site, the music starts playing as you read some pages of certain chapters.

Here is a live video of them performing Kara's song Mister. 

Lastly, this is one of the cool songs Ma Ri sings in chapter 60 :D

www.myspace.com/bandriddim
club.cyworld.com/bandriddim