Luceid is one of the 4 Guardian Lords that rule over the Guardians themselves and therefore over Filgaia; they are the Guardian of Desire. Luceid is usually depicted as being the last Guardian who can still manifest itself as a physical presence and has appeared as different genders in its different incarnations.
In the events transpiring before the first Wild Arms, Luceid is the Guardian that sided with the Demons, especifically the Demon Boomerang. This was brought about by Boomerang's desire for battle, which is the greatest desire that could be found on Filgaia. It was also because of Boomerang's desire for battle that Luceid's power never decreased, and why Luceid has been able to maintain a physical body as opposed to the other Guardians. No rune exists for Luceid for that reason. In the events during the first Wild Arms, Luceid takes a female gender to compliment Boomerang's gender. The pair challenge the protagonists in several parts of the game. In an optional side quest nearing the end of the game, Luceid has been transformed into a Guardian Blade which posesses his/her entire strength that Boomerang uses as his ultimate weapon in his ultimate form, Boomerang Flash. In the remake (Wild Arms: Alter Code F) Boomerang hands over the Guardian Blade to the party before his death once defeated and Luceid changes from a blade to a rune so that the party can summon her into battle.
In Wild Arms 2 Luceid has changed genders to male and fights alongside Anastasia Valeria in her quest to defeat the Blaze of Disaster, Lord Blazer, before the events of Wild Arms 2 and disappears from Filgaia along with Anastasia once she successfully seals Lord Blazer away. During the game's present timeline it is revealed that Luceid and Anastasia were sent to a sort of purgatory upon the end of their quest. Anastasia explains it by saying because they were remembered, they never fully died. When Anastasia is successful in returning Ashley from this purgatory, she also sends Luceid to watch after him. In an optional side quest Ashley can meet Luceid again in the dungeon Werewolf's Den. When Ashley calls out Luceid's name he is teleported into a sub-space of sorts and confronted by Luceid. Lucied tells Ashley he must have a good, strong desire in order to make use of his/her powers. When Ashley tells Lucied of his desire to create a world where a hero isn't needed, Luceid lends him his/her strength, replacing Ashley's "Hot Fencer" force ability in his Knightblazer form with "Mad Luceid", making the attack much stronger.
In Wild Arms 3, Lucied goes from a character involved in the storyline to a typical summon. Like the rest of the Guardian Lords in the game, you must defeat Luceid before you can use them.
In Wild Arms XF, NPC dialog establishes that Luceid is still present on Filgaia and is wandering the planet attracted to sources of strong desires. It is possible that Tony (Wild Arms XF), a seemingly unimportant white dog who travels with the main party, is actually Luceid; the game does not confirm or deny this and leaves it ambiguous.