I already have two trainers so I thought 'Why not make a gym leader oc?'...maybe I'll make an Elite Four OC...but that seems pretteh tough, they're always pretty out there on pokemon types and designs...O.O
He specializes in Dark-type pokemon. I chose his pokemon based on a 'boss' kind of theme. Like those pokemon that seem like crooks, mobsters, etc.. Who are still pretty cool, though. He's not particularly mobster-ish, more like a super super rich young man.
I imagine his gym would be a mansion with gems everywhere which will set of some sort of trap to make you get lost and have to battle other trainers.
I wanted to make a Psychic-type gym leader, but there are already too many of those, so I chose Dark-type, which there are none of. Dark and Psychic types are my favourite, it's a shame they cancel each other out, though...XD