Haha I am sure it's just an oversight, but it's fun to pretend it is intentional (not just in Suikoden, but in games in general haha). But yeah, Sheena was actually quite a good character in Suikoden 1 - his main issue was that he had the lightning rune permanently attached to him, which combined with Suikoden 1's one rune limitation, limited his usability in certain situations.
On a related note, I don't remember the particular characters, but I know you can't recruit Oulan if you bring certain female characters for the recruitment cutscene. I believe Sierra is one. This makes sense, since she is a very powerful vampire who controls a true rune, and it would make absolutely no sense to have her be a damsel in distress by a couple of street thugs haha. So it would be fun if there were an actual plot/character reason for why Sheena doesn't protect Lorelai haha.