She is a Jumi who possess a core of diamond.

Spoiler

The leader of the Jumi, Diana functions as the Guardian of Rubens. She desperately attempts to keep the Jumi from extinction with a hard-headed dictatorship, one that sees her unfeelingly exploiting the Clarius Florina's ability to shed life-giving tears at the expense of the latter's life. This, in turn, spurs Sandra's decision to betray the Jumi by robbing them of their cores, heralding the near-extinction of the race.