The SA-X is a creature in the video game Metroid Fusion. She is a parasite that originally infected the protagonist, Samus Aran, as well as her Power Suit, before Samus was cured by injecting Metroid DNA into her.

It's because the x in general and the metroids are mortal enemies. When the SA-X arrived at the battle between Samus and the Omega Metroid, It saw a powerful member of it's mortal enemy, and a weakened Samus. It just decided to first fight the bigger threat. Helping Samus was just a side-effect.

Chronologically speaking, SA-X is the second doppelgänger/clone of Samus Aran, with the first being Dark Samus.

Stick to the low ground and fire charged beam shots directly at the SA-X to destroy it. Your beam must be fully charged to damage it. As it approaches, Space Jump over the SA-X to the other side of the floor while charging your beam, and release the shot when you land.