MEGA MAN STAR FORCE REFERENCE

The Transer (heh) [...] is a mobile terminal that appears in Mega Man Star Force as an evolution of the PErsonal Terminal from the Mega Man Battle Network series. Most people wear their Transers on their arms[, but it can also be detached from the wriststrap].

The Transer is just a wearable Nintendo DS in form, because that's the platform the game is on, and instead of apps, you insert different physical cards, as the battle system has card game elements. Gimmick-based worldbuilding.

Coincidentally, it was released in Japan in December of 2006.