Shadow Zero are very small usually pitch black beings with a large eye where their face should be. They have appeared in all of the Seiken Densetsu games, but in Secret of Mana they were not enemies you could fight. By the way they act they could almost be considered doppelgangers.
In FFA these monsters made their homes in many different places and were always paired with Cobras. They could walk through any obstacle and melt down into an invisible form. Generally they will randomly float about the room, but on occasion they will lunge towards your character. Coming in contact with a Shadow Zero can sometimes blind you.
Shadows X1, X2 and X3
These shadows of yourself must be faced in the Cave of Courage. They can barely do any damage to you, and can usually only be harmed by charge attacks. Sometimes they will cast Acid Rain. If the character they are mimicking dies they will die as well.
In SoM Shadow Zero are very rarely encountered in treasure chests. The most common place to find them is in drops from enemies that frequent the Palace of Darkness, particularly Fierce Head and Dark Knights. When the chest is opened the message bar displays “Shadow Zero!” and one of the monsters jumps out of the chest, looks around surprised and then crawls back inside. What it does is unknown, but some sources say that it will completely revive a character if they are dead. That speculation would make sense for the monster, because in future SD games the Shadow Zero could mimic people perfectly. Another sighting of the monster occurs randomly when you cast a level 8:99 Evil Gate. On a successful cast the screen will be overrun by a bunch of Shadow Zeroes. When all of them run off the screen one final one will run by, lagging behind the pack.
In SD3 Shadow Zeroes are classified as Morphs.
Found in the Mana Holyland, these blue colored Shadow Zeroes have the ability to morph into different monsters as they take damage. The boss Genova also summons them to fight. They share the name Shape Shifter and the morphing ability with a Slime from SoM. When morphed into another monster they can only use their physical attacks. They should be killed as fast as possible because every time they morph they regain all of their HP.
Also found in the Mana Holyland, Shadow Zeroes are potentially the most dangerous enemies in the game. Whenever their HP hits 75% they morph into the closest one of your characters to them. They become a copy of that character and can use all of their techniques. It should be noted they cannot cast any of the characters spells. Shadow Zeroes are immune to all status effects. Their eyes can be used to cast Black Curse on enemies.
In LoM Shadow Zeroes are classified as Morphs.
LoM describes Shadow Zero as being “a small quick moving shadow with big attacks.” They can use many different punching and kicking attacks, and seem to have the ability to morph their shape as they like. As a pet their Syncro effect protects you from being blinded by enemies. After completing the “Teardrop Crystal” quest you can go to Domina on a Salamander day to find a Shadow Zero waiting in the pub. The Shadow Zero offers you a mini-game where you catch other Shadow Zeroes to gain “Zerobees” and win various prizes.
This notable Shadow Zero actually had a quest based after it, it is described as “loving to grab people’s attention.” In the quest the Shadow Zero takes the form of your character and does various mean things to characters around Domina. In the end the Shadow Zero-One professes how it is made of all of your hatred and declares that it hates you. Before the battle with Shadow Zero-One starts he states “BRING IT ON!” This mini-boss can be very challenging if faced early on in the game. To activate this quest, “Seeing Double”, you must achieve a Shade ranking of 3 in Domina.