On April Fools Day, 2021, Taiko no Tatsujin's Twitter Account posted a QR Code, along with a foreboding conversation between Don-Chan and Ka-Chan. By scanning this QR Code on a compatible Arcade Cabinet, the unique song "彁" (pronounced "Ka") by composer LeaF is unlocked. A harder Hidden Oni difficulty was distributed via QR Code the next year on the same day. The song is notable for a few reasons:

• "彁" is currently the only song in the game that is unlocked through scanning a QR Code; all other Codes provide cosmetic options for Don-Chan. Likewise, the song is added to a genre folder separate from the rest of the game's music, labeled as "Hidden Data", with the subtitle translating to "That's No Good".

• The name of the song technically does not exist. "彁" is a "Ghost Character", a character that exists in Unicode, but does not anywhere else. While other Ghost Characters have some sort of explanation for their existence, "彁" has no definitive source. Its pronounciation of "Ka" was provided by LeaF herself, as the character otherwise has none.

• Upon selecting the song's Hidden Oni chart, a unique transition effect plays with glitchy effects. No other song in the game has a unique animation for going from Oni to Hidden Oni difficulty.

• The Hidden Oni Chart is considered one of the hardest songs in the series. This is attributed to a heavy utilization of multiple scrolling speeds and polyrhythmic patterns. Only one individual is known to have full comboed it when it first debuted.