While checking my Twitter DMs briefly, I happened to catch that Masashi Kokura, the character designer for the original Tokimeki Memorial, shared a few scans of his original designs for Shiori. (Source)
This is technically not the first time at least some of these designs have surfaced--a rare book published for the game's 10th anniversary (which, yes, I own) actually includes a series of fascinating planning documents and production drawings from the game's development days, including these designs. There are more pieces in the thread linked above, but either way, you can definitely trace a lineage to how she ultimately turned out.
It's worth noting that prior to Tokimemo, Kokura had no experience drawing anime girls of that ilk whatsoever (bishoujo, if you will). As the story goes, after the PC Engine version launched in May 1994, he took a month off, feeling that he had failed and hadn't produced any appealing character art. It was only once he returned to the office that he became aware of the game's reception and growing success and that it turned out that, yeah, people liked what he drew!
All of which is to say, if you suffer from imposter syndrome, you're not alone: even the artist behind some of the most famous women in Japanese games has struggled with it. If the dude what created Shiori has had it, then I think we can all cut ourselves a little slack. 