Ashido is a Shinigami introduced in episode 148 of Bleach, though he remains largely enigmatic throughout the series. He's best known for being the lone Soul Reaper stationed in Hueco Mundo, where he hunts Hollows before they can breach into the living world—essentially doing the heavy lifting so other Shinigamis don't have to.
His story in Hueco Mundo started as a mission gone wrong. Ashido and his squad were pursuing Hollows that fled into their territory, and they kept fighting until the others were killed or couldn't continue. Rather than abandon the post, Ashido stayed behind, making it his ongoing responsibility to thin out the Hollow population at the source.
The most distinctive thing about him is the Hollow mask he wears constantly. It belonged to the last Hollow his friend killed before dying, and Ashido keeps it as a memento. Beyond sentimental value, though, the mask is practical—it deflects Cero blasts and functions as solid protection in combat, which probably helps explain how he's managed to survive alone in enemy territory for so long. The mask does obscure what fans note is actually a fairly handsome face underneath, but he seems to have accepted that trade-off.
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