It isn't so much that Trevor is mundane (obviously, he isn't). It's that Trevor has the kind of backstory that a D&D fighter should be representing.
This is how I think of it. Every class goes to level 20. You can quibble over the exact break points, but any character that gets into Tier 2 is capable of supernatural feats. Since class defines the bulk of a character's growth and capabilities, then supernatural ability is a necessary component of every class's kit.
If you want to skin the supernatural feats as a magical bloodline or a once-in-a-generation slayer, go for it. If you want to skin them as "strong-willed kid doesn't know when or how to quit", then you can do that too.
It's not the job of the core books to define your setting and narratives for you. D&D is a toolkit, but the PHB is not a world-building tool.