During Tsubasa's first encounter with a Maguar, a Ribers container was ruptured and the contents made contact with her. Within seconds, the Ribers fused with Tsubasa to form Figure 17, which eliminated the alien threat. Shortly afterwards, they separated and the Ribers molded itself into an identical copy of Tsubasa and called itself "Hikaru."

Aside from physical appearance, Hikaru shares the same memories with Tsubasa. After D.D. brainwashes Tsubasa's father to hide his and Hikaru's identity, Hikaru goes to Tsubasa's school as her twin sister.

In contrast to Tsubasa's shy nature, Hikaru is lively, talkative and outgoing. Consequently, this slowly helps Tsubasa become more open to everyone around her.

(Source: figure17.areaseven.net)