Overview
Sei Asagiri is an HP Shield buffing HG.
Sei Asagiri does not apply HP Shields to all allies, only to “frontline” Dolls and Fairy summons. While her HP Shields are actually quite decent, she doesn’t bring anything else than this, making her generally inferior to other options. Sei Asagiri’s interactions with Sei and Jill are not strong enough to consider teambuilding around.
With this in mind, Sei Asagiri is a potential HP Shield buffer, but other options tends to bring more (higher uptime, buffing the entire Echelon or other effects). As such, Sei Asagiri is optional at best.