Kyle, open
Kyle didn't precisely understand the concept of Halloween. He understood what he was meant to do for the party part of it, and he'd found a costume that he liked in the store. He was dressed in a pair of white shorts with a small sheet wrapped around his middle. A sash crossed his chest that read "Happy New Year" to inform the others that he was baby New Year.
Since he spent most of his time wandering around in pajama pants anyway, having his upper body bared was no big thing to him, and by now, nearly everyone had seen his lack of a navel.
He looked around to see if River was there, though if she wasn't he reasoned she'd show up eventually ... unless something had distracted her. In that case, he'd wander up to go find her at some point later.