Who is the Pyro?

Interview with Engineer

Engi: Pyro, huh? Well now, I can't tell ya much, since almost nobody can, but I sure as hell can tell you more than the average man.
Mr. Deep: Thank you, Engineer. I-
Engi: Please, you can call me Dell.
Mr. Deep: Alright. Dell, what is your take on the identity of Pyro?
Engi: Believe it or not, but I've seen men like Pyro before.
Mr. Deep: Really?
Engi: Heck yeah. Workin' in the Texas oilfields fer' ten years will teach a man a thing or two. Now there are two kinds of folks in the world; the people who work with others, and the people who roam alone. Pyro is in the latter.
Mr. Deep: You say Pyro likes being a loner.
Engi: While I'm tendin' to my sentries, I rarely see Pyro move with the other guys. He's always ahead or behind them. And sometimes, he'll even move back to behind the allied lines. Now other folks who are more concerned about kill counts would call him a yella' bellied coward, but I know better. He's hunttin' Spies.
Mr. Deep: Makes sense. Spies often tend to work behind the scenes.
Engi: His overgrown blow torch also seemed to be like a Spy repellent. Even our own Spy avoids him when possible.
Mr. Deep: Do you help him?
Engi: Heck yeah. I help him find those backstabbin' snakes-in-the-grass and he keeps my buildings safe from sappers.
Mr. Deep: Interesting. Sounds like Pyro actually helps his teammates.
Engi: Yup, and anybody who tells you otherwise is lying like bloodhound.