“I never asked anyone to understand me! This is my own business. It’s the connection between me and Harukappa. For that, I’ll pretend to be anything!”
-Kazuki Yasaka, Sarazanmai, Episode 1
Naturally, some of the questions inspired by Kazuki Yasaka’s outburst is of Harukappa, who they are, and what Kazuki’s relationship or connection to them is exactly.
The short answer is that Harukappa is Harukawa Yasaka, a kid who lives with Kazuki and is likely Kazuki’s younger sibling.
(a few mild spoilers for Mawaru Penguindrum, specifically Episode 9)