D is a bioroid, an artificially constructed biological/mechanical being first introduced in the Tenchi Muyo! universe. The war between the Earth Defense Force and the Rara Army began when D and at least two mechas were discovered in an artifact of unknown origin. Her original body was damaged, so a bioroid was built to contain her essence (represented by a glowing orb in place of an eye) while her body healed.

D appears to be as inhuman as the robot she pilots, speaking mechanically and asking many questions about human emotions and behavior. The only one to break through her stony exterior is Kazuki, to the shock of Commander Sanada. D frequently has visions of a horrible creature attacking, and is deathly afraid of Himiko.

She develops a strong attachment to Kazuki, culminating in her decision to return to her real body at the climax of the series in order to save him. It's revealed that D stole Zinv and fled her world to escape Himiko and Kumu, who claimed the lives of her friends.

As a robot pilot, D is subpar, often requiring rescue by Kazuki and Mitsuki. However, she redeems herself by removing the limiter placed upon Zinv, allowing its true power to manifest itself.

When Zinv is destroyed and the worlds merge, D becomes Dee Sanada, taking on the animated personality of a young girl.

There is also an apparent resemblance between D and Tenchi Muyo! GXP character Neju na Melmas, leading some to believe that she and D may belong to the same race.

(Source: Wikipedia)