Oh, I think I saw what Suika was trying to do! Notice how Aya said '1?' in panel 3? they're trying to pin Rin under the balloon! Or, perhaps it's more of a distraction to make them THINK that they're trying to pin her? Either way, Yuugi either foiled it or lead herself into their trap.
I think that's a katakana "ko" (コ) - ie, a coughing or choking sound. It would be pretty odd to start the count just after you've called a foul, after all.
Fo...So am I~What a coincidence...Foul~!Mugg!Episode 505: Bad. This is BadFrom the Artist's Pixiv pageTotal Bursting...What~?!*long blows~**long blows~*This is too reckless!This is way too reckless!M...Move...*struggles*The air balloon has filled the barrier!I am weak to these kind of things.Wow~*pierces*