Ridiculously Average Guy: Compared to Agito, Risker, and Valcuria he's extremely average, being a student while the other three have had combat training...it makes his acts of heroism and his level in bad ass all the more dramatic.
Revolvers Are Just Better: Depending on the media, averted and played straight. In the manga for one scene he wielded a revolver, it did nothing to the Zoanoid. However the Buster Gun has a rotating chamber holding only three bullets so played straight there.
What If?: Some fans see Aso as being an older and tougher Tetsuro, and Toki as Sho if he hadn't gained the Guyver and grew up.
The Ace: Considering he's a bishounen who looks to be 20, apparently immortal, the strongest person on the planet arguably, built up a secret organization, and rules over the world, he's a dark version and leads to..
Broken Ace: He may have a sympathetic back story that fleshes him out but he's pulled off quite a lot of stuff...also he's slowly dying.
And I Must Scream (He was rooted in place on a tiny isolated island with no one to talk to but his own semi-intelligent extensions for 'A HUNDRED THOUSAND YEARS'. It's no wonder he's fanatically loyal to Dr. Barcas.)
Dark Is Not Evil: Extremely loyal to Barcus and explaining if not for him he would never learn self respect or more about the world.
Heroic Sacrifice: An antagonistic version to help stop an arguably worse person, Agito