Flash Sentry

FS refers to a character in My Little Pony Equestria Girls.

Development
In late April 2013, an image of Twilight Sparkle and this character as they appear in the film, with this character holding a guitar case, was posted online bearing the file name "ts_fs.png" and accompanied by the text "MMM HMM".

Quotes
"Whoa!"

- Trailer

"You okay?"

- Trailer