Cero Doble

Cero Doble, meaning Double Zero in Spanish and Heavy Duet Hollow Flash in Japanese, is a reflecting variant of the Cero belonging to Nelliel Tu Odelschwanck. Rather than simply firing energy outward, it works by receiving a hostile attack first and answering with a stronger one.

The technique begins when Nelliel takes an incoming Reiatsu-based strike into her mouth, absorbing it whole. She then discharges her own Cero at the same moment she releases the captured energy, and the two combine into a single reinforced blast. In her diminished child body the redirection is possible but exhausting; she manages to bounce back a Cero from the released Privaron Espada Dordoni Alessandro Del Socaccio, yet the strain leaves her spent. Restored to her adult form, the fusion grows far more devastating, since her own output blends fully into the rebounded attack. According to Tite Kubo, the swallowing trick is not limited to Cero and can also catch and return things like a Getsuga Tenshō or various Kidō spells.

Nelliel is the technique's primary user across the manga and anime. In the light novels the move spreads further: Roka Paramia copies Nelliel's data through her Resurrección and reproduces it, though the power overwhelms her body when she gulps down Cien Granz's Gran Rey Cero. Nelliel herself is also written rebounding a Gran Rey Cero in another novel, an effort that leaves her mouth bleeding.