@Vicic
A lot of the personality for Isabel is leeched out in the translation. For example, she refers to herself in the third person as 소녀 often but the translation just skims right over that and tells you that she speaks formally. She also calls Frey 귀하 often, but that's just translated as "you" which is the most generic translation you can have.
소녀 for example is "maiden" in this context. It has a Japanese equivalent for the sake of familiarity since many of us here have heard it: "shoujo." In fact, 소녀 (so'nyuh) and shoujo are written using the same Chinese characters: "少女." It's a title that deliberates lowers herself while she raises Frey with 귀하 (gyhi'hah) which is "honored self." I have no idea why the manhwa says she talks in military jingo because how she talks is in no way how anyone in the military talks, except maybe (maaaaaaybe) they sometimes have a way of talking in the third person in the period dramas, like "This maiden solider requests your honored self's permission to engage the enemy."
Anyway, from how she talks and how the author framed it, we can impute that she comes from a highly regimented, strict family with a strong military involvement and she sees herself as a junior in how she deals with others. Thing is, the author isn't terribly consistent with how he handles this character. If he sees a chance to break character for a cheap joke, he takes it 9 times out of 10. So any flaws you see in Isabel is directly attributable to the author and the story as a whole.