In his post on Nyoron Churuya-san 04, Jaalin riddles us with a screencap and tooltip "if you don't already recognize her, the voice will surely do it".

The only thing that comes to mind is the snowbug from Aria, but, as far as I remember, it does not speak.

I'm out of ideas. And yes, the voice does seem quite familiar.

UPDATE: Hikago says it's Kyon's sister (CV: Sayaka Aoki, credited for both roles).