A kerbonaut is the KSP equivalent of an astronaut. Kerbonauts are Kerbals who pilot manned spacecraft, space planes, rovers or any other not probe controlled craft.
Each kerbonaut has two attributes which are shown in meters next to its name in the vessel info page and in the astronaut complex, which show its courage and stupidity. These values affect how the kerbonaut reacts to decouplers decoupling, parts blowing up, and other events. There is a third attribute, Bad Ass, which overrides courage and stupidity. Jebediah, along with a few other kerbonauts, has this flag enabled.
The three starting kerbonauts are Jebediah “Jeb” Kerman, Bill Kerman, and Bob Kerman. More can be hired in the Astronaut Complex, each one having a random name, courage, and stupidity. The surname of any kerbonaut is always Kerman.
The Original Three
Every new game, whether Sandbox or Career mode, begins with three kerbonauts already available and ready for launch. While all other kerbonauts have randomly generated names and stats, these original three are always the same: Jebediah “Jeb” Kerman, Bill Kerman, and Bob Kerman.
These are the only "veteran" kerbonauts in Kerbal Space Program, designated by orange IVA suits; everything else is visually identical to all kerbonauts. In some of the game's loading screens, three orange suited kerbonauts (presumably Jeb, Bill, and Bob) have helmets with orange stripes instead of red but this isn't seen in-game.
Because they are hired by default and always listed first in the same order they are most likely join missions than any of the random kerbonauts. The best chances to join is the kerbonaut first, which is usually Jeb followed by Bill and then by Bob.