Super Mario Odyssey will let a second player control Mario's hat via local co-op as reported by Polygon.
The news came about during Nintendo's Treehouse event - a livestream which goes into more details about their games. The stream goes for more than 28 minutes, but you can jump ahead to the relevant segment regarding co-op by clicking here.
Mario's hat, aptly named "Cappy", can attack enemies and pick-up items for Mario. So while his role may be secondary to that of Mario's, Nintendo said that was meant to be the case:
"When you throw Cappy, he's invincible, so we thought that would be a really good opportunity for people who aren't use to games to be able to play with someone else who's a little more advanced," said Kenta Motorua, the game's director.
He also explained Nintendo's inspiration behind the inclusion:
"We know we wanted Mario to be able to throw something, and then we thought, 'well, he's always wearing a hat', so if he just throws his hat that'd be great. And once we got to that point we realised people can have more of an emotional attachment if it was also a character; then once we realised it was a character, he should be able to move around himself, and so the multiplayer element really just feel into place."
Super Mario Odyssey will release October 27 for Switch.