This manga is too ridiculous. Looks like they're not going for a serious tone and more of a comedic tone. Her saying that she will lay her life down for her lady also probably means that in future we're going to get an arc where she has to choose between MC and her lady and she will be facing this huge conflict.
There is also another huge conflict
Development is at warp speed not complaining, gotta wonder if she met Masamune in the past or something for her actions to make sense or its just because she's a woman of culture