Metroid

Metroids are the titular species of parasitic aliens from the Metroid franchise. Since Super Smash Bros. Brawl, Metroid has become a recurring Assist Trophy in the Super Smash Bros. games.

Super Smash Bros. Melee
In Super Smash Bros. Melee, there is a trophy of Metroid. It can be obtained through random means, such as from the Lottery.

Super Smash Bros. Brawl
In Super Smash Bros. Brawl, as an Assist Trophy, Metroid erratically flies around the stage. It occasionally emits a screech before charging at a fighter and latching onto them. In doing so, the Metroid slowly leeches that fighter's HP. After a while, the Metroid gets off the fighter and resumes its random flight around the stage, looking for another (or the same) fighter to latch onto. After doing this a few times, the Metroid vanishes in a plume of smoke.

The game features a trophy of Metroid and two stickers, one of which depicts its Metroid Pinball artwork and the other its Metroid: Zero Mission artwork. The former sticker enhances freezing attacks while the latter sticker enhances electric-based attacks and is only usable by Samus.

Super Smash Bros. for Nintendo 3DS / Wii U
Metroid returns as an Assist Trophy in Super Smash Bros. for Nintendo 3DS and Super Smash Bros. for Wii U, retaining its behavior from Super Smash Bros. Brawl. Both games have a shared trophy of Metroid.

In Super Smash Bros. for Nintendo 3DS, Metroids are enemies in the Smash Run mode. For the most part, their behavior is the same as their Assist Trophy counterpart, except Metroids in Smash Run can be defeated, causing them to drop a lot of Stat Boosts.

Super Smash Bros. Ultimate
Metroids return as an Assist Trophy in Super Smash Bros. Ultimate. Fighters can defeat a Metroid, although unlike Smash Run, Metroids can only be defeated from freezing attacks. There is a spirit of Metroid, which can heal damage caused from a KO.