All of the core KND's names are allusions to their numbers:
Numbuh 1 is Nigel Uno, the last name being the Spanish word for one.
Numbuh 2 is Hoagie Pennywhistle Gilligan Jr., making him the second of his family to bear that name.
Numbuh 3 is Kuki Sanban, her last name being the Japanese word for three.
Numbuh 4 is Wallabee Beatles, his last name being the same as the famous British rock band of the late 1960's, also known as the Fab Four.
Numbuh 5 is Abigail Lincoln, her last name being the same as President Abraham Lincoln, whose image is on the U.S. five dollar bill.