- Rick himself: an electronics wizard and all around nosy fellow;
- Scotty: the husky former Marine, Rick's foremost pal;
- Barby, Rick's sister;
- Jan, daughter of a scientist, Barby's best friend, and Rick's crush;
- Dismal the dog;
- Chahda: the Hindu boy, mastermind, and good friend of Rick and Scotty;
- Dr. Zircon: the barrel of a scientist;
- Julius, the mathematician;
- Steve Ames, JANIG agent;
- Hartson Brant, chief scientist at Spindrift Island, and Rick's dad;
Hal Goodwin, under the pseudonym John Blaine, wrote most of the stories. Hal was a rocket enthusiast, and that showed up in several stories, such as the Rocket's Shadow (hitting the moon with a rocket, in 1947), the Scarlet Lake Mystery (rocket building in the desert), and the Flying Stingaree (UFOs and seafood on the east coast).
The stories are well done, and I heartily recommend them. They are mostly stand-alone, but every now and again something from a previous plot will show up.
Incidentally, the first books in the series are fairly easy to find, because so many were printed. Grosset and Dunlap kept reprinting the first ones as new volumes came out. There were few printings of the last books in the series, however. Those stories are rare and can fetch several hundred dollars (or more) if in excellent condition.
Also interesting is that the Rick Brant stories largely inspired the classic Johnny Quest cartoon adventure series, and those are a lot of fun too.