First I want to explain that to me a "Trap" in the context of Anime/Manga fictional characters is a mostly Cis male character who dresses like a girl and is able to pass as one. I prefer not to use it of characters who I think of as Transgender or even Non-Binary, (which I feel Anime definitely needs more of) and that includes the character in Steins;Gate who I wish had been handled better. Now some might disagree on the Cisness of Traps, and that's fine I fully support fans reinterpreting characters however they want. But as far as Canon goes I don't think the character in Twin Angel Break would boldly declare "I'm actually a Boy" if that wasn't his preferred Gender Identity.
Trap characters have been important to me as a mostly Cis-Het Male (though I've recently considered that I may be a little Fluid) who still doesn't confirm to Gender norms easily. I would dress more femininely if I felt I could look decent doing so. Before I got truly deep into Anime the fictional characters I identified with and liked most were almost always female. And since going full Otaku they still tend to be, but the characters considered Traps have been the refreshing change.
The word "Trap" is not truly being used literally, the fandom generally means it affectionately. And Western media has a long history of even the slightest gender ambiguity being a sign of depravity and villainy that still lingers even today, and has popped up sometimes in Anime and Video Games. But the characters that define the Trap archetype have always been morally solid characters, sometimes including Astolfo the most honorable and good hearted on the show.
And now to what people actually came for, what is distinct about Astolfo at least among the ones I've seen thus far? Making him not better but certainly notable.
It's that he is still for a Trap pretty "traditionally" masculine. For example while other Traps might get involved in actions and fight scenes from time to time, they're not usually a type of professional solider.
Now I know it's sexist to say that is an inherently masculine straight, but that's the point. Traps often aren't truly questioning the Gender binary because they'd just boys who look like girls by every external appearance, both physically and in personality. They're very traditionally feminine often more so then the Cis-Women of the show.
Astolfo however is kind of the counter part to what I like about Sailor Jupiter/Makoto Kino/Lita. I'm too not fond of the first Sailor Jupiter post I make on this blog given some of what I've learned since. The gist is she's a character who at first glance seems like the designated Tomboy, but she does have many very traditionally feminine traits. And some would argue Minako is the true tomboy, which has it's own poetic irony to it.
Basically Astolfo, Makoto and Minako are characters who are not simply an attempt to swing the pendulum of gender norms hard in the opposite direction, but rather are nuanced individuals.