Heya! I read a few chapters in Japanese some time ago (before I decided that the kansai dialect is a tad too difficult to understand) and (at least in those) both Yoshiki and Hikaru's names do not use any kanji, so Yoshiki is just よしき (and Hikaru is written with either hiragana or katakana to symbolise if it's the real Hikaru or "Hikaru")
As for the possible kanji, it can be written in many ways and combinations
I'd be curious to know if the author ever mentioned the specific kanji used!
How do I spell Yoshikis name in kanji? I'm trying to figure it out but I'm lost