Psychology:
Reiko prides herself first and foremost on her professionalism. She tries to keep her work life and her private life separate, which usually results in her coming off as cold or disinterested to most people she interacts with. While working, having fun is secondary (if on the list at all). Efficiency is important to her, and adhering to command structures are essential. Still, even the most solid structure has its weak points, and Reiko's demeanour can be shaken. Sometimes angry, other times concerned, these are the moments when she herself starts to come through. She will usually attempt to brush these moments off, or take them in stride as if they aren't even there or were planned. She takes a long time to "warm up" to anyone, and even when she does, she is still brusque with them at times. She prefers to be a second-in-command over a leader, although she has strong leadership abilities and can command and organize people well. In all her interactions, she will treat people on a need-to-know basis. People she is close enough to can try to wrangle more information from her, although they have to get past the obvious block of "necessary silence" to do it. Her motivations are her own, and she will very rarely reaveal much about herself.
Her strong sense of determination and dedication are her primary strengths. She sees things through to the end, regardless of people coming and going along the way. When something is worthwhile, she can dedicate herself to it until it's completed - regardless of what comes up along the way. She considers herself better as a follower than a leader, even though she is quite capable of assuming a leadership role (and is essentially forced to when it comes to the Chosen Ones). These beliefs lead to an almost fanatical following of her superior Takumi, whom she addresses as "Master Yata" while working, and she trusts his words and takes his orders without hesitation or question. In fact, her following of Takumi can at times estrange her from others, as she will withhold information or even act against the common goal if instructed to do so. She can count the number of people she actually trusts on one hand, and always keeps everyone at arm's length. Thus she has few true allies or friends and a load of mostly useless acquaintences.
A programmer and engineer by trade, she has a very logical mind. She can see through patterns and puzzles very well, and even in odd situations can work through things rationally. She prefers to think things through and plan, and to cover as many angles as possible. She has a quick mind and good reflexes, though when the pressure's on or she's presented with something outside the "parameters" of what has been planned she may get stalled. If a plan doesn't work as predicted, she tends to take the pessimistic view of the failure even if valuable information was gained.
There are times however, where some darkness begins to leak through. Due to the continued usage of her Avatar and occasionally its special ability, parts of her subconscious have begun to surface. Oftentimes she may become quite moody or exceptionally prone to violence or aggression. She will lash out at those around her either verbally or physically. During these times, she may become downright sociopathic as the anger within her spills out. It is these times where she will forget her self-imposed vow of ensuring that GU's purpose ends successfully and begins to seek her own revenge. She is aware of this when it happens, and accepts it as a side-effect of the path she chose to follow. Due to this tendency to act out the drives of her Avatar, she limits her interactions with others when possible in order to avoid trouble. She is most comfortable with other Chosen ones, especially since they all understand the difficulties that they have and basically don't care if someone is Expressing.