He belongs to a special category of summons that take multiple people and a lot of time and preparation to summon successfully ( And the one time it is summoned it kills all of them from the strain, including a l'Cie), but once he does appear, he wipes out the entire enemy military in the spaces of about 10 seconds. His appearance in Type 0 is also worthy of mention.
Then IX made him the ultimate Eidolon capable of taking out Bahamut, and he's since been regarded alongside Odin and Bahamut as one of the most powerful summons.
Light 'em Up: There are several Holy elemental summons across the series like Maduin and Ultima, but Alexander is more recurring and famous.In XIV, he's so massive that 3 entire raid series take place within it's body. In all of these he's a story-line summon only, and can't be used in battle. Humongous Mecha: Is always one of the larger summons, taken to the most extreme degree in IX, Type 0, and XIV, where he's the size of a castle and takes more than one person to summon and in World of Final Fantasy where he's big enough to serve as a bridge that spans a massive desert between two countries.Holy Hand Grenade: It's usually of the Holy element, when that element appears in a game.Good Wings, Evil Wings: A few games give him a massive set of angelic wings.Guardian Entity: Described as one to all mankind in some games.While he's not one of the more talkative summons in any appearance, he's just as conscious in all of his appearances. Genius Loci: He's generally depicted as a city-sized summon, and several of his incarnations (most famously, Final Fantasy IX) are used as a residence.Alexander is heavily tied to the Holy element while Ark is tied to the Dark element. Alexander is a massive fortress with limited mobility while Ark is a transforming airship with high mobility. Both are incredibly powerful mechanical based summons. Foil: Ark/Cruise Chaser from IX was created to serve as this to Alexander.
Final Boss: In of the Treasures of Aht Urhgan expansion in Final Fantasy XI, Final Fantasy XIV in his eponymous raid series, and in Bahamut Lagoon as a dragon.Games switch between whether this is an overwhelming series of blasts, one giant blast from a laser cannon, or both. Energy Weapon: Alexander's light-based attacks are typically shown to be from the turret of the summon's parapets.Big Fancy Castle: Usually depicted as a mecha that is shaped like one.