Aura's sister and twin cousin of Aoba's twin brother, Sei Seragaki.

She was known as "The Resurrected One" because when she and Aura were born as one, sadly they had died. But after separating the two, she was revived immediately.

For anyone confused about the four:

Aoba = 'Death'

Sei = 'Life'

Aura = 'Reincarnate'

Sia = 'Ressurect'

That's what they symbolized