Any Servant (such as a Ruler-class) who can see the statistics of other Servants will notice that Astolfo's profile has been heavily edited. He has access to powerful magic through his Noble Phantasm "Casseur de Logistille: Destruction Declaration," which allows him to perform feats of magecraft. Rather than using this to his tactical advantage, he instead graffitied his statblock with rambling remarks and doodles. He also covered his gender, preferring to let people guess for themselves on that matter.
Astolfo has great respect for King Charlemagne and considers himself a knight in "Charlie's" service above all else, even his existence as a Heroic Spirit. Nevertheless, they have a uniquely terrible sort of chemistry wherein they bring out the worst in each other: If left unsupervised, they will undoubtedly get up to mischief. Although Charlemagne considers Astolfo an idiot (in the gentlest and most affectionate usage of the term), Astolfo is shockingly adept at convincing his King to do stupid, dangerous, and otherwise fool-hardly things, purely for his own amusement. It's for the best that eleven other paladins were present to dilute their chaotic behavior.
Speaking of other paladins, Astolfo is biologically related to two of them: Bradamante and Roland. His hippogriff originally belonged to Bradamante, one of his cousins. He found himself in need of flight during his journey, so they traded with one another; Bradamante received Rabicano, his magical horse made of hurricane and flame, and his enchanted lance, Argalia. Because his most famous adventure involved flying to the moon while hitched to the aforementioned hippogriff, the Throne of Heroes recorded it as his possession, and determined his class to be Rider. Bradamante, in turn, was recorded as a Lancer. She does not seem overly thrilled about this. If Bradamante were to be summoned as a Rider, she would also have ownership of the hippogriff, but Astolfo seems incompatible with the Lancer class (though his primary weapon is indeed a lance).
Astolfo's relationship with his cousin Roland is unusual. Though he proudly wears the ribbons Roland gave to him in life, he also accuses his fellow paladin of being an out-of-control pervert with a number of alarming fetishes. He seems almost in awe of Roland's degeneracy. Roland, in turn, speaks to Astolfo like he's a child. One could say they're more like brothers engaged in sibling rivalry than cousins.
In Fate/Grand Order, Astolfo is classified as a genderless Servant. He is immune to skills, Charm spells, and Noble Phantasms that target men and/or women specifically. He does, however, have the "Appears Feminine" trait, so Blackbeard's Gentlemen's Love skill will restore his HP. This is the only skill in the game that impacts Servants with the "Appears Feminine" trait.
Astolfo's first major story appearance in Fate/Grand Order is during Subsingularity II: Agartha. He and D'eon are guest Servants during this chapter; however, they leyshifted into the singularity by bribing a certain staff member and are not rogue Servants confined to the singularity. In short, this means that both Astolfo and D'eon are canonically present within Chaldea during Fate/Grand Order.
[FATE/APOCRYPHA SPOILERS] Though Astolfo and his Master did not win the war for the Greater Grail in Fate/Apocrypha, they both survived the conflict. Sieg's transformation into Fafnir also turned him into a limitless font of mana. Despite being separated by different planes of reality, Sieg's mana still reached Astolfo through the veil, making him effectively immortal. He chose to take advantage of this for a time and went on a plethora of adventures, just as he did in life. Some of his journeys are recorded in the short story Fate/Apocrypha: Story of Survivor (which has not been translated into English). During this time, he visited with some of the other surviving Masters, tried and failed to reach Sieg on the Reverse Side of the World, and researched how Ruler Amakusa was able to summon Assassin Semiramis. Once he was satisfied with what he'd learned, he released himself from his Contract with Sieg to await another summoning.
