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Ririn is a modified soul created to track the Bounts. Since they no longer exist she helps out around Urahara's shop.
Ririn first appeared as an initial threat along with Noba and Kurodo. but is later revealed to be modded souls created by Urahara for the purpost of tracking Bounts.
They first appeared in the anime only filler Arc, The Bount Arc. They also appear throughout the storyline, appearing in places that they werent present at in the manga, an example of this is when Grand Fisher returns to the real form as an Arrancar and attacked Kon who was in the body of Ichigo, Ririn and the others fought against Grand Fisher to save Kon, and was later along with Kon Saved by Isshin Kuosaki where as in the manga only Kon is saved.
Ririn in her gigai form appears to be a young girl with short blonde hair, she wears a pink coat with a black cloak over, In her other form she inhibits a Plushie Bird which wears a blue cloak with a hood and a red bow tie. Ririn has the special ability of creating illusions, changing an environment. She also has the power of tracking the pressence of bounts.