True Companions/Newspaper Comics

Flash Gordon, Dale Arden, Hans Zarkov, Barin, Thun, and Vultan. Any of them would die for any of the others. Aura eventually joins, too.
Alley Oop is part of two groups. In prehistoric Moo, he's got Foozy, Ooola, Dinny, Guz, Umpa and th' Grand Wizer. In modern times, he's got Doc Wonmug, Oscar Boom, and Ava (although Oscar has been stuck on the Heel Face Revolving Door for a few years now). Both groups have met and are friends, but they don't interact very much.
Even though they get on each other's nerves a lot, the core cast of Peanuts counts.