[ earlier in the day, kate and boo went fishing ]And the quote-unquote fishing trip explains the ten-gallon fish tank now in Kate's room. And the silver minnow swimming in said tank.
His name, according to Boo, is Fred the Fish.
Boo, of course, being Concerned and Responsible, has to ensure that Kate takes proper care of Fred, so she's spending the night in Kate's room.
Only now Kate's awake in the middle of the night, and there's no Boo asleep beside her.