Prior to becoming a Números, Roka was a white, spider-like Adjuchas Hollow, born from Szayelaporro Granz’ successful attempts to create an artificial Menos. At some point before becoming an Arrancar, around 250 years before, Roka was sent to the Rukongai by Szayelaporro alongside several weaker Hollows so he could observe the Shinigami in combat.
In the Arrancar arc, doing her duties as a nurse in Las Noches, Roka reattaches Yammy Llargo’s severed arm.

Trivia: This character is not named by Tite Kubo in the manga or databooks, nor is she named in the anime. The name Roka Paramia comes from the novel Bleach: Spirits Are Forever With You by Ryougo Narita.

(Source: Bleach Wiki)