Although technically an Epitaph User, Kazumi just isn't quite up to snuff in that regard. He can only surprise someone once with his limited repertoire, and after that any past/potential victims are more or less on to him. Although he can see Avatars he lacks one himself, instead simply becoming a much stronger normal person. He relies heavily on his words and his technical skills to carry him, which when taken outside of that scope leaves him mostly with just his charms. Similarly, while he's quite good at what he does, barring that he's rather limited. Everything about him is so focused on achieving his personal goals that while he can gloss over the gaps with social graces, when closely examined he falls short - although why he falls short may never be realized. Someone who never finds out the truth might just consider him an overly polite, overly friendly computer geek.
Perhaps his biggest weakness is his own psychosis. While he understands the dangers and risks of what he does, he simply doesn't care. To him, the ends justify the means. When focusing on his goal he often loses sight of the big picture. While it's fine to point out that reactivating the RA Plan will probably trigger Cubia, he's quite aware of that and wants Cubia to bring it regardless of what happens to himself or the system around them. He has a complete lack of true empathy, and thus is for the most part inhuman when finally revealing himself. He has issues with betrayal, as well, and does not take kindly to those who he deems cowards, or those who back out of things for "foolish" reasons. Those who do not share his desire to take whatever risk necessary to complete a task are pathetic, and barely worth more than rage, especially when their actions directly counter his own. His past is his biggest demon, in that it has consumed him to the point where there isn't anything more to him. Because of this, those who manage to learn the truth behind him may also learn the key to his undoing.
Also worth mentioning is his manual clumsiness. Due to his hands being oversized, and his fingertips being sharp talons, he lacks a bit of dexterity. He has difficulty with fine manipulation or holding small objects, making certain tasks such as writing or eating with cutlery difficult. He also lacks the ability to feel with his hands, which can be beneficial in regards to extreme temperatures, means that he can't judge things apart from how much resistance he's encountering. Also, things like typing present troubles due to having pointy fingertips. He will go through quite a few keyboards until he learns how to control his touch...