Zipper (ジッパー, Jippā) is a ring Dimensional ÄRM that it allows the user to store his or her ÄRMs in an extra-dimensional space for storage. When activated the user can unzip it and reach inside and grab whatever was previously stored. The main advantage of it is that it can be used in battle for a fast change of ÄRMs.
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Zipper in the manga.
Zipper's ring in the manga.