Seseri

Seseri is the half-sister of Ageha and Tateha, and an antagonist in the Espgaluda series of bullet hell shoot 'em ups. She appears as a boss twice in the first game, and once in the second. In Espgaluda II Black Label's Black Label mode, her 12-year-old form replaces Asagi as the third playable character.
 

In Espgaluda

Seseri was fathered by king Jakou with an unknown woman after the queen had sealed herself away. As the queen's blood did not flow through her veins, she does not possess supernatural powers and is not eligible to participate in the Espgaluda project. Due to her father's obsession with her half-siblings' abilities, she grows jealous, and when their place of hiding is found she volunteers to participate in the hunt. She is defeated by them once, returns with bandages and mechanical parts for revenge, and is then defeated again.

In Espgaluda II 

Somehow, Soma's engineers managed to rebuild her, though she appears to now be entirely mechanical. She once again fights and loses against her siblings, who are now aided by Asagi.