After the Metroid capsule was stolen by Ridley, the Space Pirates conducted numerous experiments on the Baby Metroid, extracting its DNA to clone new Metroids and exposing it to various forms of radiation. The Baby was unable to form a bond with its captors, despite the Space Pirates’ efforts to build a body for Mother Brain it could relate to, and the Baby became fearful and aggressive in its strange new environment, attacking anything that moved. The Baby was relegated to the heart of Tourian, where it could be more easily controlled by Mother Brain’s telepathy and kept under close observation. The Baby consumed large amounts of energy from Zebes’ strongest life-forms, including the Ultra Side Hopper and Torizo, which caused it to grow immense and become the Super Metroid.
Though controlled by Mother Brain’s telepathy, when the Super Metroid encountered Samus, the bond with its “mother” proved too strong, and the Baby flew off. When Samus was about to be destroyed by Mother Brain, the Baby came back, draining Mother Brain’s energy and refilling Samus’. However, Mother Brain revived and killed the Baby; its remains fell upon Samus, releasing massive amounts of energy and unlocking the Hyper Beam, with which Samus laid Mother Brain to waste. With that final act of self-sacrifice, the Metroid species became extinct...
None. The Super Metroid cannot be defeated. Simply let it latch onto Samus, and it will drain her down to 1 Energy. There is a Missile and Energy recharge station two rooms over. Later during the Mother Brain fight, the Super Metroid will return to save Samus. It will refill Samus’ energy, and its corpse will endow Samus with the Hyper Beam, the Baby’s final gift.
Though the Super Metroid and Metroid Fusion instruction manuals read “the Metroid hatchling”, the original Japanese text calls it the Baby Metroid. It was not until Metroid: Other M that series director Yoshio Sakamoto insisted on the translators calling it the Baby.