personality
Reina is not a character with an easy-to-discern personality. She keeps so much of herself guarded and lets us observe so very little. There are however, some distinct traits we see surface time and again. Here are just a few of the elements that make up Reina's personality (to the best of our knowledge).
brilliant
Reina excels in absolutely everything she does. Her brilliance across all fields is no surprise — she was designed to be this way. Reina, as a first generation Neo-gene, was genetically hand-picked to be superior to a normal human. Furumizu and the NSWF made her to be of genius-level intelligence, physically fit like an athlete, and beautiful like a model. Takayama notes that Reina's across-the-board brilliance is what endeared her to him when they first met all those years ago:
She was an outstanding woman. Intelligent, refined, physically fit and beautiful. Everything about her.
Reina is the most powerful and beloved of the Neo-genes because she ticks every designated box. Her intelligence allows her to pioneer research, while her athletic skill facilitates her Cloneblade abilities. Does this mean that Reina is, in all facets, brilliant? Well, no. Reina's "special" aspects (in both body and mind) were fostered through bio-engineering and a strictly controlled education/upbringing. In making Reina a mind and body without equal, Furumizu neglected the parts he didn't regard as important — things like her capacity to socialise and love.
While Reina is exceedingly intelligent and her personality reflects this, her ability to express anything beyond what she has been engineered to express is severely limited.
Selfish
Following on from the above, as she is not well socialised, Reina is terribly selfish. In fact, she has no real regard for how anything she does impacts on others. There is nothing intentional about this behaviour (she is not sociopathic), but it is quite clear that she acts wholly for herself through the vast majority of the series. This is probably best displayed in her relationship with Takayama.
The relationship between Takayama and Reina was a mutual experience built between the pair. Her decision to use Takayama as a means to conceive a child however, was purely selfish. To Reina, the conception of Rihoko was another way to test her abilities as a Neo-gene. When Takayama discovers that Reina's child is in fact Rihoko (episode thirteen), he tells Segawa that the child's conception was more than likely an experiment: "She probably wanted to assess things on her own, including children". She did not consider for a moment whether Takayama wanted a child, nor his feelings on the matter after the child was conceived. From beginning to end, falling pregnant was about Reina and no one else.
In the present day, the doctor is equally as selfish. When she discovers Rihoko is alive, she dismisses herself from Furumizu's side and opts to pursue custody of the child she'd assumed dead. This selfishness is a little easier to relate to, but it is still a strong display of Reina's inherent self-orientated drive. Indeed, reestablishing contact with Rihoko has nothing to do with the child's well-being and everything to do with Reina wanting to test herself as a parent before she dies.
It is only after spending actual time with her daughter that Reina begins to see things in a different way. Having Rihoko makes her consider the well-being of someone other than herself for the very first time. Reina's veil of selfishness is ironically lifted by the little girl she'd selfishly conceived for her own observation all those years before.
honest
Reina is brutally honest with both herself and others. This frequently extends to a lack of tact on her part. There is no sugar-coating in Reina's world, not even for those she does not know well. When something important is on her mind, she says it, like: "Controlling the Witchblade is impossible for you" or "Shiori died because of the Cloneblade. I may end up like her in time". Reina is particularly honest with herself after her breakdown commences. She is aware that her time is running out but refuses to delude herself about it, "My own Blade-induced breakdown appears to have begun. End report". Sonozaki Mie said of Reina's bluntness:
She's very honest about what she's thinking or feeling.
For the most part, Reina never outright lies to other characters, although she can be selective with what she says around Takayama and Furumizu. When Reina does lie directly, it is to protect Rihoko (such as her "It's all right. I'll be back for you." lie, which she herself cannot fathom). Overall however, Reina is a woman of direct honesty.