Other characters that sound like the idols?

[ninjamitsuki]

After listening to more of Makoto's speaking voice in videos (I don't have Perfect Sun, though I want it because I like Makoto), I found that she kind of sounds like China/Wang Yao from Axis Powers Hetalia. Well, I guess it's more like China sounds like Makoto because im@s came before the Hetalia anime.

[animagic4u]

I would mention Toradora...But they are the same voice actors. 

[BT2]

Renton's vocal range doesn't exactly sound like Sanpei's female voice type so that one doesn't really count imo.
That and using the same seiyuu's kind of..

If you really wanted to mention Sanpei and/or some other character who appeared in a certain iM@s game at the same time you should've posted this..
http://www.youtube.com/watch?v=-AA931M96rg

[iM@Sotaku]

The twins in Vocaloid [Kagamine Rin and Len]
are voiced by our own adorable twins! Ami and Mami!

[iM@Sotaku]

Also the voice actress for Iori is very popular.
As the voice of Al from full metal.
Taiga from tora dora
and that one main chick from the familiarity of zero!
and many many more!

[animagic4u]

Ryuuguu Komachi reminded me of the trio from Toradora when they sing together but in both cases I think Rie Kugimiya's voice is quite the prominent sound.