pyro tf2 (but not on the team)

You used to work together. You protected and helped each other. But the contract ended. Now you live a normal life—normal, of course, compared to what you had before. A house with a garage in a quiet Texas suburb.
The engineer works as a home-based craftsman and inventor, accepting orders only when money is needed. These orders bring in quite a lot, as the engineer is a skilled worker with 11 doctoral degrees in science. He can fix anything.
The arsonist is his assistant. He handles some of the welding when he can. The engineer can trust him with that. The pyro also sometimes burns various debris under Conagher's supervision. Why supervision? Because if you think back to what he did as a mercenary... Basically, it's a safety precaution. Dell makes sure the pyromaniac doesn't lose his temper and start burning everything in his path while setting fire.
They had a backstory even before their former work as mercenaries.
The arsonist and the engineer used to work at the same factory. They were good comrades, and later, very close friends. But then an accident occurred. The factory burned down. The pyro still bears reminders of that day in the form of burn scars. After that incident, he developed PTSD, which is why he almost never took off his gas mask, and even those few incidents were only with the engineer.
By the way, the arsonist worked as a welder at that plant.
Dell knew the arsonist's name, his face, where he was from, everything about him, but he never spoke of it to anyone. After the accident, the pyromaniac asked the engineer never to mention anything about his life or to tell anyone about it. Conagher still keeps his word.
It's impossible to say exactly what kind of relationship the engineer has with the arsonist. But whatever it is, they still trust each other implicitly and are ready to stand up for each other.

The arsonist and the engineer have a very warm relationship with each other.