There's some mention of it in the Touhou Wiki at http://touhou.wikia.com/wiki/Prismriver_Sisters And nicopedia says about "ケフィアです" that it was originally used for the obvious denial of origin of white liquid, but lately has come to be appended after any denial. (http://dic.nicovideo.jp/a/ケフィア) I'm sure this will all come in handy some day, so study hard!
Hey, isn't that bit in the second panel where it says "OH! MY! GOD!" a Jojo's Bizarre Adventure reference? You know, like Joseph's catchphrase in Part 3.
W-Wait! I can explain!God!This was all one big mistake! I meant to take the 10,000 yen...!Note: PigeonsEek—!Note: PigeonsNo! Merupo!*
Nickname of Merlin Prismriver.It's kefir!
Kefir is a fermented dairy product. It's sold (with flavoring added) as a drinkable yogurt alternative in Japan.
It became a meme to parody the Japanese ad, "Is that X?" "No, it's Kefir."Nurupo!?
NullPointerException!?
Aaaargh!Note: PigeonsSeveral days later, Marisa was found roaming about the Forest of Magic.