If you have stayed with us across the headlining period for 3DS3, you should know about the endless, slightly random promotional campaigns with recent Nintendo console Taiko releases after launch, which involves you going to various game-selling stores to get additional swag.
For this iteration of that quote-unquote "trend", the swag in question is the new Performance Character of Platinum Don-chan (プラチナどんちゃん). To get the character, you should visit participating stores where new promotional materials are displayed starting today (November 29) and access the QR code on them. That should give you the download code with which to grant the additional content. The campaign (display of promotional materials) will end on January 13 2019, and the download codes will expire on March 31. Worry not of the expiry dates (too much) though, as there are still opportunity that it may be later sold as paid DLC.
While we haven't found actual shots of them in-game after the release, we have heard from an earlier leaked source (who wishes not to be named here) that the Performance Skills are Spirit Gauge Start at 50% (魂ゲージ50%スタート) and Drumroll Note Support Lv. 2 (連打音符サポート Lv.2).
Campaign page on Nintendo Switch Version main site