Igarashi plays a character called Shinji Noe. Sadly, I never read the manga much so I have no idea or may have forgotten who this guy is. (But whatever, I like the pic in this blog post anyway.)
Mizushima plays a character called Minami Nanba. I think I remember him a little. Seems like an outrageous guy. (And thankfully, from the appearance of this character, Mizushima's hair would probably return to the way I like--long, wild, out of control.) I must say, after Hatsukare and Lovely Complex, this is the third time I'll see him in a show based on a shoujo manga.
Shirota plays a character called Makoto Kagurazaka. No idea who this guy is either. According to his blog, though, he is the main character's rival.
Yamamoto plays a character called Taiki Kayashima. I think I should know who he is, but I apparently forget. (This is getting somewhat tiring -_-, so maybe I'll go read the manga.)
ETA: This scan is from Drama CD 2.
Ha ha. Apparently Noe looks like this (middle guy on bottom row). I'm almost sure I've never seen him before. Kinda deflate me somewhat. However, Nanba (top right) is looking better than I remember. :) And Kayashima is the pretty(?) guy on the left.
This page has a picture of Kagurazaka and another picture of Noe.
ETA 2: ::Beats self up and down with stick:: How can I even proclaim to like Yusuke Yamamoto (Kabuto's Bocchama) and completely, completely missed him in the cast list? Even worse, his name is right below Mizushima's too. More, more incentive to move this drama to the top of my 'to be watch' list.