Ruka (or “Luca” as she is called in the English version of the anime) is a girl who serves under Sonoka. She has a sadistic personality and has the power to control water and wind, however she states her specialty is electricity.
She has a sadistic personality, and in the manga often ‘flirts’ with Sumire while they are fighting. She has the power to control water and wind, but she states that her specialty is electricity.
Ruka has long purple hair and blood red eyes. She wears a purple dress with fuzzy yellow trim around her shoulders and on the tops of her stockings.
Ruka first appears in Chapter 8 of the manga when she attacks Yoshiki and as a result convinces Sumire to let Lucia's guardian train her so she can learn to control herself in beserker mode and be able to protect people she cares about.
In Chapter 19, we find out that Ruka is working for Sonoka because she was saved from having her own fragment stolen (which would have turned her into a virus.) If Sonoka hadn't helped her, she could have died. When Sonoka found her, she had just had her shard ripped out, but she still had her human consciousness. In exchange for saving her life, Ruka agreed to help Sonoka in her cause. It was at this point she was named Ruka by Sonoka, we have no previous name for her.