She is Aoba's classmate, apparently she is from the year 2088 who came into Aoba's time in order to protect him from the Great Zogilia forces who came from her time to kill him. During her time travel to Aoba's time, she brought with her own Valiancer mech to fend off the Valiancer that came to kill him. An event she has prior knowledge about from Aoba in the future.
It was later on revealed that in the year 2088 she belongs to the Great Zogilia Forces which are at war with the Confederate Treaty of Liberty Alliance and that her name is Hina Ryazan.
In the final episode it is revealed that Hina Ryazan has been caught in a time loop, perpetually travelling backwards and forwards in time to protect Aoba. Thanks to a memory transfer, both Dio and Aoba realise this and put a stop to the loop.
During her flashbacks during the OVA's it's revealed that Hina Yumihara is originally from the present, and attended college with a future Aoba. She eventually participates in the initial stages of the Coupling System development, which causes her to be sent to the year 2088 inside the cockpit of Luxon. From there she eventually gets sent back in time to Aoba's high school days and protects him when her enemy chases her down from the future. This event sparks the time loop, as she and Aoba are forced to flee into the future to survive, she regresses back to a child and loses her memories. Getting adopted as Hina Ryazan. Aoba then takes her place as the pilot of Luxon.
Her iconic hairpin is from this timeline, where it was won at a festival stall for her.
In the final episode we see her return to the initial Coupling System test, restoring her to her correct timeline.
When Aoba returns to his timeline, he makes a chance encounter with her in a way reminiscent to how she met him in her timeline. It is unknown as to what impacts this has on their respective future timelines.
