Defen

Defen (also known as Defen the Invincible) was the final boss of the Deadly Six Stars in chapter 6 of Metroid EX. A lizard-like being, he dropped Rastodon, a massive acidic sea monster into the pure water of the ocean world and turned it into a bog of sulfuric acid. He possessed the Data Capsule containing Samus Aran's stolen Varia Suit, which protected him from the acid.

When Samus lands on the planet and ventures off into town, Defen attacks Joey Apronika and Diesel, her comrades, and traps them in a submarine, which he drops into the water. Samus is warned about a "Sea Demon" that is responsible for the water toxicity and assumes Defen is this monster. The submarine is incapable of rising to the surface and is only strong enough to remain intact for five minutes. Samus jumps in after it in an attempt to save them, but is attacked by Defen and Rastodon, the true acid monster. They wound her and leave her to die on the ocean floor, but Samus triggers a dormant volcano to launch herself and her friends out of the water. Defen and Rastodon are killed in the explosion, but Samus is unable to recover her Varia Suit, because its Data Capsule is lost in the explosion.

Trivia

 * Defen's name is an anagram of "defense", which is the Varia Suit's signature trait.