That's actually an oddly hard to answer question. Out of the three most recent stories, I'd take the one I'm mainly working on for this. Normal Knights and Guards aren't weak per se, even if there's stronger ones around. I'd originally say the younger stepsister of the MC, but she's still a noble, so she technically still has more power than others. If we go just by physical power, yes, she's weak, so would be her mother, MC's stepmother, who's a Grand Duchess and holds a lot of power.
I guess I would choose one of MC's maids. MC has a handful of servants, but only two are regularly around her. They come to turn quite loyal and turn from her closest aids to sort of friends she happens to employ. One of them gains actual abilities and becomes a force of her own, but the other remains a regular human. She's important as someone almost always around, yet she doesn't fight, because she clearly can't, and she's a maid, so she doesn't hold a noble title of her own either. On all accounts, I believe she might be the weakest character in every manner of power.