I think a big reason I'm slightly insane about Barney Calhoun is because he's so... ordinary. There's technically nothing special about him. He's a college dropout, his only armor is a helmet and a vest, he's got basic firearms training and some maintenance knowledge and that seems to be it. He's just Some Guy who likes drinking beer and reading about conspiracy theories and secretly keeping a pet toad in his locker.
He didn't even start out as his own character– "Barney" is the nickname that playtesters gave to all of Half Life's security guards, and Valve ran with it. It was retconned that our Barney appears in the opening sequence of Half Life 1 for a few seconds, but he's identical to every other guard... because technically, he IS every guard.
He exists to pass on messages, to open doors, to lighten the mood, to drop ammo or weapons that the player can scavenge, and to die. Crushed by debris. Gunned down by the military during a coverup. Bleeding out inches from a med station. As we learn at the beginning of Blue Shift, that's literally in his job description at Black Mesa: his personal safety is the lowest priority, placed even below the preservation of equipment. He's a cog in the machine, and the machine has deemed him expendable.
As someone else here on Tumblr pointed out, even the G-Man (who keeps tabs on all of Half Life's protagonists throughout the series) seems uninterested in Barney. By the end of Blue Shift, G-Man appears to have stopped watching; he doesn't even care enough to find out if Barney survived, and writes him off as "out of range" and likely deceased.
In the face of everything, Barney seems insignificant. Gordon's the one out there blowing things up, pissing off powerful entities, and inadvertently becoming a hero with a near-mythical reputation. And yet, whether it's working undercover in Civil Protection, or acting as a Resistance field commander, or simply giving an old friend a crowbar... Barney's impact, while going unseen by the wider world, is essential. He knows how to play the part of a cog in the machine, and he'll help bring down the system from the inside.
Gordon and Barney play complementary roles to each other, down to the colors of their uniforms. Gordon is the right man in the wrong place. Barney is the wrong man in the right place. It probably wasn't even intentional on Valve's part, but man, I canNOT get enough of it