I was re-watching A Crown of Candy and noticed something (spoilers):
In Episode 3, when Ruby is inside the carriage after the ambush, she casts Prestidigitation to clean the blood off her flyer. Calroy Cruller reaches in and grabs her hand to interrupt the cast. Brennan described it as "swashbuckler dexterity" that allowed him to do that.
Rogues don't have an ability to react to spell casts like that. Swashbuckler subclasses don't either.
You know what grants that ability? The Mage Slayer feat:
When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.
Brennan threw a red flag out there so early. Holy shit.