A friend is someone with whom you have a positive relationship. In good friendships, this positive relationship goes both ways, with each party getting equivalent benefits from associating with the other. In bad friendships, one party might take advantage of the other without giving back in return. It is wise to avoid these.
Agents somehow manage to strike up friendships with each other, despite there generally not being too much time for them to socialize. The existence of hangouts in HQ such as the PPC Lounge, PPC Bleepka Bar, and Rudi's attests to the determination of many agents to make the time.
"Friendly" is also a good word to describe PPC Boarders (if the person communicating with them is likewise).