Well you could say she was Mika's Sister but not quite. This is the second Personality of Mika Hatsukiga. This personality is all of what Mika desired; like lust, killing, strength, and power. She was originally the Beta Form Of Mika, A power given to Mika from Hikari Takeshi, the Creator of the Great Clans of the Forest. Rika had chose this name because of the fact that She was getting sick of people calling her Zeta Form Mika.
At one time it was called Beta Form Mika, but due to an experiment by Koudo, It is now Zeta Form Mika. Still Rika Hatsukiga Though. Rika has all of Mika's powers but they are stronger and a lot faster. The body that she is in is easily changeable, But she doesn't know that. Rika can freely leave the body and enter it to be with Mika. When she goes back to her host body,( Mika's body), She can go into the Zeta form.