Though not mentioned in game, Mari is the heir to the KounatRoyal Family, which was destroyed when Ashtaroth lost control of the Aernas Hammer, a legendary weapon only usable by the "Geas" imposed on the Kounat Royal Family, making Mari its "key." Mari was put in an escape pod which failed to activate properly due to the explosion that annihilated Kounat, but was saved by Void for his own purposes which are still unknown. The explosion caused the pod to malfunction and was put in hibernation mode in order to protect the passenger, though how she awakened is still unknown.
When Mari awakened as Geas after she touched the Soul Stone and recovered her memories, she protected the Grand Chase with a force field before shattering the Soul Stone, disappearing in the resulting explosion, with only her earrings left behind as the remainder of her existence. This is seemingly canon as Sieghart comments on her disappearance in the Wizard's Labyrinth and the Monster Train 301. Mari reappeared, unconscious, in Kounat's Collapse after Ashtaroth was defeated due to the Aernas Hammer's response to its "key."
Although her age is listed as "unknown", Mari is approximately 600 years old, as Kounat's destruction happened around that time since Sieghart, who is officially labeled to be 600 years old, is one of the few survivors of Kounat.
Mari was based on the Elsword character Eve. Much like Eve, Mari descends from an ancient Kingdom and uses machinery to aid her.
Mari's Wing Drive Weapon was based off of the Elsword character Raven's Arch Enemy.
Mari possesses heterochromia and is the only character with that trait.
There is much evidence that considers Mari a deity. Geas is defined as "someone who was exceptionally gifted into communicating with the divine" and "one who was a step closer to the deities than everyone else". Kounat was also founded by hybrids of humans and deities, and Mari is a member of the Kounat Royal Family.
In her pre-release in the Korean Server, she was seen wearing a robe asking who she is, what is going on, and needing help with something.
Strangely, in her dialogue before she joins the Grand Chase, her identity is masked with "??????". This does not apply to any other character on their first in-game appearance.
She is one of three characters that has her full name known from their initial release, the others being Ronan (Erudon) and Elesis (Sieghart). However, neither Elesis' or Ronan's middle names are known, or whether they have one.
Mari is one of three characters that specializes in machinery, the other being Lire and Rufus.
Updates to Mari's classes have been frequently late and delayed compared to other characters. Geas was released 2 years after the release of Mari. Rune Caster Skill Tree, released 4 years after the release of Mari, only consisted of a Shared tab; it took another whole year after the Shared tab's release to implement the full tree.