I know what they changed him to in another game, but in IX (his only main series appearance) Ark is the dark foil to Alexander.
Both are transforming mechas; Alexander from a castle, Ark from an airship.
Alexander is the most powerful Gaian eidolon. Ark is the only known Terran eidolon.
Alexander deals light damage (presumably). Ark deals dark damage.
Alexander's attack as per series tradition is "Holy Judgment." Ark's attack is "Eternal Darkness," or for a literal translation from the Japanese version, "Dark Fate."
Alexander is earthbound. Ark is skybound.
... etc.