Magikarp is an Pokeball. It is rare in most stages, but common in Mélancolie Path.
When activated, Magikarp appears and uses Flail on the nearest opponent, damaging and stunning him for some time. Then Magikarp splashes once, hard on the ground, shouts, "Karp!" then leaves.
Use it at an opponent falling, where this assist is the most effective. It is also useful for characters like Phoenix Wright, who needs time to progress to a higher level and become stronger.