This gamemode was added as part of the Adventure Time crossover.
In the Buddy gamemode, teams of up to 2 players each compete against each other. Each player selects two different Legends before the game starts. During the match, each player will control both of their selected Legends at the same time. This means that all attacks are mimicked, and if a player picks up a weapon both Legends get a weapon. Players earn +2 points for getting a KO and lose -1 point for getting KO'd. The team with the most points by the end of the match wins.
The only two maps available in Buddy are Under the Canopy and Kung Foot Arena, which are both very small maps with no way to get knocked off the map. Because of this, the way to score KOs is very different - if a player gets bounced around the map enough from knockback, they'll be KO'd. If one of the player's Legends gets KO'd, only that Legend will be KO'd, not both.