Astolfo is specifically from this Fate spin-off with its own separate cast, not the mainline Fate/stay night storyline.
Popularity exceeds typical spin-off characters, largely due to an androgynous design recognized even outside the Fate fanbase.
No — Fate/Apocrypha functions as a reasonably self-contained entry point within the larger Fate franchise, not requiring the mainline story first.
Largely due to a distinctive androgynous character design that became widely recognized even among people unfamiliar with the franchise's dense lore.