It's kind of complicated, I know, but that is essentially what's happening.

The fiancee of main character guy is her close friend/maid servant and introduced her to sword fighting, and her father was expelled to a corner of the kingdom for a crime the main girl learns about later on. So there are no other high born lords or ladies around.

She's constantly getting new clothes because her dad spoils her too much to make up for taking her away from her childhood friend/soulmate. And she, while living with her father, was always rejecting the new and fancy clothes anyway so he doesn't question her when she starts going out while wearing maid outfits without the aprons, which are slightly baggy so her body muscle goes unnoticed. He thinks she's just mad at him.

And she wears main guy's old clothes during training, and excuses herself from the house by saying she's going with her maid servant friend to ogle at the hero when she's actually training with him. On days that she can't get away with this, her and her maid servant swap places and the maid acts like she's sick in bed.

A few people in the household are in on the entire situation and help the noble lady get away with this all the time, hiding it from her father.