I'm going to give you an honest reason for why I dislike Flash Sentry. Even though I say this, I'm likely gonna get at least 1 dislike from a butthurt Flashlighter. With that said, let me break it down for you.

To start, his introduction in Equestria Girls was rushed in my opinion. There wasn't much time for us all to get to know this guy. Starlight Glimmer made a sudden appearance too, but at least we got to learn a few things about her. Flash, however, really seems to have no given character other than he's the cool guy who can play guitar.

Second, I think his presence was just unnecessary. I mean, come on. Did the movie REALLY need a love interest? I personally didn't think so. There was a perfect amount of drama: The element missing, and the safety of Equestria at stake. That's it. I think the movie could've spent the time on building more drama, or learning more about this bizarre new world that Twilight has entered, rather than having Twilight make goo-goo eyes at this guy we just met.

Anyways, that's just my opinion. Do you agree? Or disagree? Let me know down below.

Alternative Reason: Damn, friggen' Flash Sentry tryin' to steal my waifu. Fucking douchebag...

I think that Flash is just a bland character. Bland being an understatement. And that being if you ignore him being the love interest for Twilight in the first two movies.
If we count that, he becomes even worse.
Why? Because, other than for how rushed and awful his interactions with Twilight were, the pair itself doesn't really work.
Mostly because their "love" is just as fake as most if not all the couples that fall on what I like to call "Doppelganger love interest" AKA a characters that falls in love with someone who looks just like somebody else.
If there is something that I hate is that kind of romance because, let's face it, is not honest and is, in fact, insulting (to the people the character has fallen in love with) and pathetic (for such character to be attracted just by how a person look). Flash didn't cared that the Twilight he was trying to impress was another one just as Twilight didn't cared if she was going out with pony Flash or not.

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I agreed and if they were going to have a love interest in movie it should have been with sunset,

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I agree. I don´t really get that whole thing.