Mario Kart: Double Dash!!'s best known feature would be the double character system, where two characters are in the same kart. Since Kirby and Ribbon have been teased by Nintendo to be a romantic couple at the end of Kirby 64: The Crystal Shards, I was thinking that they would be a default team if paired together in Double Dash!!
Character Stats
Weight: Owing to their small size, Kirby and Ribbon would be lightweights.
Special Item: This would probably be the Star Rod, which Kirby used during his duel with the Nightmare Wizard during the final battle in Kirby's Adventure. The user fires a star projectile straight ahead, and this projectile follows the course, spinning out any racers that collide with the projectile. The projectile dissipates after one lap.
Karts: Kirby and Ribbon would likely have karts that reference the Kirby Air Ride game. Kirby will use the Star Speeder, which looks like his Air Ride machine in Kirby Air Ride, albeit with wheels that look like his wheel transformation with the Wheel ability. Ribbon will have a heart-themed counterpart to the Star Speeder, titled the Ripple Racer, with the sharp point of the heart shape pointed forwards and the curved lobes of the heart shape pointed backwards. Due to their weight, these karts would have good acceleration with low top speeds, and can drive through the off-road zones without slowing down all that much.
Special Animations
Victory
Front: Both characters jump up and down happily in the driver's seat.
Back: Kirby can perform his victory dance for his victory animation if in the back, while Ribbon waves to the crowd, much like she does in the bad ending to Kirby 64: The Crystal Shards.]
Quotes
Kirby: (Unused victory jingle from Kirby 64: The Crystal Shards.)
Ribbon: "We did it! We won!"
Not Quite Perfect
Both characters just use their regular character animations.
Quotes
Kirby: (Death jingle from Kirby's Dream Land 3)
Ribbon: "Eh, that wasn't too bad."
Defeat
Front: Kirby closes his eyes and cries, much like not getting anything in Kirby 64: The Crystal Shards' goal game. Ribbon does not cry as hard, but holds one hand to an eye.
Back: Ribbon turns around and sobs like most other characters, while Kirby gets popped into the air and lands awkwardly in the back seat with his eyes crossed out, mimicking his death animation in the Kirby games.
Quotes
Kirby: (Death jingle from Kirby 64: The Crystal Shards)
Ribbon: "Oh, no!"
Miscellaneous/Other
When hurt, Kirby gets crosses in his eyes, while Ribbon's eyes become spirals.
Kirby's Quotes
Falling: "Eeyaaaah!"
Hurt: "Poy!"
Throwing Item: "Ha!"
Scoring Hit: "Poyo!"
Ribbon's Quotes
Falling: "NOOOOO!!"
Hurt: "Ow!"
Throwing Item: "Take this!"
Scoring Hit: "Yeah!"