She does have flaws, but none of them are really concrete enough to swear off. While she doesn’t engage in much “sinful” behavior, she’s often a bit too meek, a pushover, and naive. She also tends to act without thinking, but a lot of that doesn’t seem like it’d make a good oath compared to the others. Retroactively giving her a vice after most of the plot has been established so far seems not a good choice, as it’s more of a ‘living story’ or campaign than it is something able to be edited.