He may be meager and timid, but Hiyamin shows true potential. Previously living a simple, humble life in the Human Village, he was allured by the mysterious Kazuki Murasaki shortly after the latter began efforts to restart the Murasaki Family.

Hiyamin joined up with this developing ragtag crew that would become the family, intent on making his own life as a human villager less mundane and dull, and learning to build up courage and be able to stand up for himself. His motivations amused Murasaki, unusual for a typical yakuza... Not that he was a real yakuza back home anyway, but had great interest nonetheless. Murasaki would eventually take in Hiyamin, and gave him the title of Captain, with promises to be his sort-of mentor.

Hiyamin is also rather fond of Flandre Scarlet.

The reason why he goes by this awkward name is really because his parents couldn't decide which of their names they wanted for him... So they settled for mashing the katakana for their two name choices together.
(This is actually an in-universe explanation, since Hiyama is a mash-up of the real life likeness's username, Cretin, and Hiyama from Yakuza 1.)