EDIT: I'm back from class and now have time to finish this. XDD;
EDIT #2: I decided to add one more character cause I JUST thought of a seiyuu for him xDD;
Elle: Romi Paku
Yup, that's right. Elle-chan again. I got a suggestion from Father that Elle's voice could also be done by Romi Paku. I was actually also thinking of Romi Paku for Elle, especially after hearing her portrayal as Yuujirou Shihoudani of Princess Princess. Like Elle, Yuujirou is also a very effeminate boy who dresses in girls' clothes, so it's perfect XDD; I think Paku-san can really pull off the young, pre-pubescent boy voice very well. After all, she is also famous for the voices of Edward Elric of Fullmetal Alchemist, Toushirou Hitsugaya of Bleach, and Tao Ren of Shaman King.
as Yuujirou Shihoudani from Princess Princess (at 10:12, Yuujirou is the blonde one.)
as Edward Elric from Fullmetal Alchemist (at 0:51)
Richard: Miyu Irino
I think Miyu Irino's voice would best suit our adorable little Richard-kun here. ^^ Irino-san is often typecasted as the young male protagonist type roles. His role of Syaoran from Tsubasa Chronicle is how I hear Richard, very cute sounding and sweet. Miyu Irino is also known for providing the voices for Daisuke Niwa of D.N.Angel, Sora of Kingdom Hearts, and Haku of Spirited Away.
as Syaoran from Tsubasa Chronicle (at 4:00)
as Sora from Kingdom Hearts (at 1:27)
Oswald: Jun Fukuyama
Hyuu~! Yes, I decided that Jun Fukuyama would be best for Oswald-kun. Fukuyama-san is best known for playing teenage boy characters. Now, for Oswald, I'm thinking that it should be portrayed along the lines of either Tooru Kouno from Princess Princess or Hanabusa Aido from Vampire Knight. Fukuyama-san is also known for his roles as Lelouch Lamperouge from Code Geass, Kimihiro Watanuki of XXXHolic, and Yayoi Shioiri of Loveless.
As for his alter ego, Kurtz, I'm not quite sure yet. For now, I guess I'll say Jun Fukuyama if he uses a deeper, rougher tone for Kurtz's voice. ^^
as Tooru Kouno from Princess Princess (at 19:45)
as Hanabusa Aido from Vampire Knight (first at 3:44, then again at 6:28)
as Kimihiro Watanuki from XXXHolic (he appears right after the opening sequence)
as Lelouch Lampergouge from Code Geass (at 5:44)
Ryuko: Masaya Matsukaze
Masaya Matsukaze was the first voice that came to mind when thinking of Ryuko's voice. He just has that certain tone that fits those calm and cool, intelligent male characters. Although...I suppose hearing him as Kyouya Ootori of Ouran High School Host Club kind of helped with this decision. XDD;; Matsukaze-san is also known for his portrayal as Mikami Teru of Death Note.
as Kyouya Ootori from Ouran High School Host Club (at 10:06)
as Mikami Teru from Death Note (this should be obvious. He's the one repeating "Sakujo/Eliminate" over and over xDD;)
Ryuka: Sayaka Ohara
Ryuka is Ryuko's twin sister and like a lot of twins, they like to pretend to be each other, fool others, and go to each other's classes. A good majority of the school thinks that there's either only one of them, or they're both boys. Thus, Ryuka can't sound too different from her brother. According to one of Chris-chan's posts, Ryuka has an deeper voice more in the alto range for a female. It took me a while to remember, but I finally did manage to come across such a voice. I know that there's a handful of deeper female voices out there, but I think this one is right for Ryuka. Sayaka Ohara is known for playing a wide variety of characters, but both she and I like her performance as Yuuko Ichihara of XXXHolic best. According to wikipedia, Yuuko is her favorite character. ^^
as Yuuko Ichihara from XXXHolic (at 5:25)
Kevan: Daisuke Namikawa
Oh Kevaaaaaan-kun..our resident flamboyantly ghey boy...XDD;
For this character we definitely need someone who can portray that silliness and fun. Instantly, I thought of Daisuke Namikawa's portrayal of Fai D. Flourite from Tsubasa Chronicle. He's cute, fun, silly, and possibly gay, so it all works out XDD;
as Fai D. Flourite from Tsubasa Chronicle (at 15:35)
and extra video of the 'Ultimate Hyuu' and more Tsubasa silliness xDD;
*looks up at entry* Hmm...I guess that'll do for now. I can't really think of seiyuu for the remaining characters at the moment. I'll continue it another time then. ^^