Pre Vault Kaoru's worst trait is how absurdly reckless they are. They think they can take the whole world on by themself and actually believe they can handle it no problem. Thats part of why their personality shatters so hard after the Vault. They were proven wrong.
Post Vault Kaoru's biggest fault is isolating themself and bottling their emotions to try to hide their depression and abysmal mental health from their friends. They don't recognize it when they're reckless but they know they're isolating but can't seem to stop.