Flash Sentry is a member of the Royal Guard. He was tasked to find and arrest the Diamond Dogs, who stole the Element of Generosity, in Ghastly Gorge. The player meets him halfway through the dungeon, where he asks the player to deliver medical herbs to two injured guardponies. He will give you the first herb; the second herb can be found at the very end of the area.
After the player defeats the Diamond Dogs and gets the Element of Generosity, one of two cutscenes will play.
In the bad ending, the Diamond Dogs will escape and Flash Sentry needs to track them down again.
In the good ending, Flash Sentry captures the Diamond Dogs before they are able to escape.
If the good ending is obtained, a bonus area called the Training Grounds will be unlocked. In order to get the good ending, the player has to rescue both guardponies by delivering the herbs.