You can dress as King Neptune, a king, kitten or a knight.
Karate Kid
King Tut (or just a king in general) a knight kat kathren
King DavidKu-Klux Dave
You could dress up as a killer or a kid.
kaftan, kilt, keyboard, kick-boxer, king, kinky, kiss, kitkat, kitten, knife, kitchen...., kite, kiwifruit, KKK, kleenex, knitting, knee braces, know-it-all, knowledge, koala, kohl, kookaburra, kooky, kylie
Something you sit on that begins with K is your kiester.
knif
Kilt
Kia
Killers are bad and start with "k".
kool-aid
Ka-ching!