Character: flash sentry
Hide yo kids. Hide yo waifu. Dey stealin' e'rrybuddy up in hyear.
Flash Sentry, as he appears on the FiM series is a royal guard with an orange coat and blue hair who occasionally makes eyes at Twilight Sparkle, but we all know him as the Waifu Stealer from the teen high school musical drama Equestria Girls. He got this reputation by sharing romantic moments with the transmogrified Book Horse in the afore-mentioned feature-length trope, thus causing twifags to completely lose their collective shit.
Human Flash Sentry (known affectionately as Brad by horsefuckers) is a walking cliche of a dudebro high school student. He spikes his hair, drives a sports car, plays an electric guitar without an amp or power cables, dates the prom queen, and wears a black leather jacket (best not to tell Twilight what leather is). He falls for Twilight for seemingly no reason.
More inexplicable is Twilight's attraction to Flash. She is an adult; he is a teenage boy. She is a horse; he is a human. It is an odd, and questionably ethical, ephebo-bestial attraction. Actually, come to think of it, that makes perfect sense for a bookish nerd like Twilight.
But back to Flash...there really is nothing more to say about him. His personality is even more of a baked potato than that of Shining Armor. The Cook/Price/Breckel "A-Team" at IDW gave Flash Sentry a couple of cameos in their comics in which they gave him an understandable fear of oppossums and in so doing made him as interesting as he ever has been.