Shadow Zero is a character that has appeared in all of the Seiken Densetsu games. In Final Fantasy Adventure, they were common enemies that upon touching the player would blind them. In Secret of Mana, they appeared in treasure chests rarely and did nothing. They also randomly appeared when casting a lvl. 8:99 Evil Gate spell when they would storm the screen with one lagging behind the pack. In Seiken Densetsu 3, Shadow Zeros only appear in the Mana Holyland and once its HP drops to 75% it shape shifts into one of your characters. In Legend of Mana, a Shadow Zero can be obtained as a pet which protects your from blindness and offers a mini-game in the pub in Domina. A special kind of Shadow Zero can be fought called "Shadow Zero-One" as a mini-boss which shape-shifts into your character.
The Mana series (known in Japan as Seiken Densetsu) actually started off as a Gameboy spinoff to Final Fantasy, called Final Fantasy Adventure. The Final Fantasy elements the game originally had were dropped in the remake, Sword of Mana for GBA.