Super Smash Bros. for Nintendo 3DS/Assist Trophies

Assist Trophies are listed here and like Poké Balls will release something to help you. Only one Assist Trophy and the character it releases can be on the stage at any time.

Prince of Sablé
Upon summon, he poses, and transforms. Depending on the transformation, he will move differently.

If a rival touches the Prince during either transformation, Sable will pummel the fighter in a classic cloud of dust, dealing 1% many times. The fighter is stuck in the cloud, so it can allow the summoner to follow up with a strong move. Otherwise, if the fighter evades the Prince long enough, he will revert back into his human self and either transform again, or disappear.

Starfy
The prince of Pufftop! He is actually not too strong, and he can be knocked around by attacks. On the plus side, his signature Star Spin can trap fighters within it with pretty surprising priortity, allowing the summoner to chain attacks to the move. He can also jump, and his odd movement can confuse fighters.