Barney Calhoun is one of the main characters of the Half-Life series. He was a security guard at Black Mesa before becoming a key Resistance leader. From Half-Life: Blue Shift and onwards, he turns from the Half-Life generic security guard into a standalone character, is given a surname, and an expanded role.
In the story of Half Life 2, this is a guy that always has your back when he's around.
Without him, Gordon Freeman would surely be lost.