Not all know the story of Ace and BOI.
I'll fill you in.
He was said to show up in the fifth Equestria Girls movie as the main antagonist.
Supposedly EQG 5 will be about the Equestria Girls government noticing all the strange anomalies happening (just like Sci-light did at the end of EqG 2 and in EqG 3) along with the strange reports they've been getting and sends some young looking special agent named Ace Thruster with a magic nullifying device in to infiltrate the high school to find out what's going on.
He's not alone, as he has a fellow agent named Coffee Klatch working with him and is his wingman.
Both are from the Bureau Of Investigation (BOI).
BOI thinks that Equestria is evil and trying to take over the world by sending in magical creatures and using magic to corrupt people.
They want to destroy the portal or take the fight to Equestria.
TL;DR: An agent of the goverment dispached by the organization Bureau Of Investigation to stop the menace of Equestrian magic.
One of his quirks is not being impressed by legit magic but liking card tricks or something like that.
Some fear him because they think he sounds like the ultimate Chad.
Sometimes it's said he'll have a flying powersuit(think kamen rider) or flying horse themed mech to neutralize particular dangerous enemies.