Idk is a character based off of a Toby Fox sketch sent out in the official UNDERTALE/DELTARUNE newsletter. Currently, very little is known about this character. Idk is a bright colored crewmate with a blue visor and small horns. It turns extremely muscular in its up pose. The character comes from a comic Toby Fox made where he depicted himself as an among us crewmate. It faces off against The Kid(a stylized version of Boyfriend) in its namesake song. Idk.

More facts here: https://fridaynightfunking.fandom.com/wiki/VS._Impostor#undefined