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Yamcha: Yamcha was trained by Master Roshi so he is another fighter that knows the Kamehameha.  Yamcha is the first Z fighter that Goku meets.  The first time Goku gets beat up is by Yamcha in the desert.  Yamcha saved everyone from Pilaf and oozaru Goku.  Yamcha beats up Invisible Man.  He beat Recoome, which Vegeta, Krillin, and Gohan got beat up by, on King Kai's planet.  He flew Goku home when he had the heart virus.  He also helped in beating Cell by distracting him.  He didn't even stay down after Cell knocked all of them down.  Yamcha used to be on a professional baseball team. 
Tien: Tien was trained by Crane Elder.  His little buddy is Chiaotzu.  Tien was Chiaotzu's body guard when he was an emperor.  The way I see it Tien is the strongest human.  Tien and Chiaotzu were the first to fly.  He beat Goku to win the 22nd World Tournament.  He invented the Solar Flare, which is used by many people.  He put up a good fight against Nappa.  He helped fight Piccolo Daimao.  He tried using the Mafuba technique, but it didn't work and he almost died.  He stops Imperfect Cell with his Tribeam to save 18.  He also saves Gohan from Buu.  On King Kai's planet he beats Jace and Burter at the same time.  Tenshinhan is the strongest human.  He also helped defeat Perfect Cell by distracting him. 
Chiaotzu: The former emperor was also trained by Crane Elder, Master Roshi's rival.  He is relatively weak, but won't hesitate to fight if Tien is in trouble.  He detonates himself on Nappa's back.  Unfortunately, it kills the little guy and not Nappa.  He also helps in the fight against Piccolo Daimao.  On King Kai's planet he beats Guldo, which Krillin and Gohan had trouble with.  He has a wife in the Mystical Adventures movie.  Chiaotzu is also a superb cook and was one of the fists to be able to fly..
Master Roshi: Master Roshi is a legendary master.  He has trained Yamcha, Krillin, Goku, Ox King, and Grandpa Gohan.  He won the 21st World Tournament, as Jackie Chun.  Roshi invented the legendary Kamehameha Wave.  He even blew up the moon once.  Roshi helped Tien turn good.  He was one of the warriors who fought Piccolo Damao.  He tried to seal him using the Mafuba.
Turtle: I put Turtle on the site because no one seems to like him, but he is cool.  He keeps Master Roshi in line.  He even beat two guys up after he saved Maron.  Turtle introduced Goku to Mater Roshi.  He celebrates his 1,000th birthday during DBZ.
Yajirobe: Yajirobe's biggest acomplishment was when he cut off Vegeta's tail in the Saiyan Saga.  Yajirobe is the person who usually brings senzu beans to the Z-Soldiers.  Once he carried Goku up to Korin's Tower.  One of the funniest things he's done was with Cymbal.  Goku ate his lunch so he sliced Cymbal and ate him.  Yajirobe adds humor to the series as well.
Oolong: Oolong hasn't done much, but did do one big thing.  He wished for panties so Pilaf couldn't wish to rule the world.
Puar: Puar is Yamcha's life-long friend.  Puar also hasn't done much, but has done more than Oolong.  When Bulma was a carrot she turned into a bird and snatched Bulma from Boss Rabbit's hand.  After that she scared the rabbit by turning into him and threatening to touch him.  She also cut off Goku's tail when he was in the oozaru form for the first time in DB.  Puar and Upa defeat Dracula Man, who had beaten Krillin.
All of these characters (except Nail) played bigger roles in DB.  In GT they were virtually non-existant.
Kami: Kami was the guardian of the Earth for most of the series.  He fought Piccolo Damao in Dragonball.  He helped train Goku, Yamcha, Tien, Chiaotzu, Yajirobe, and Krillin.  He merged with Piccolo and gave him a lot of strength and wisdom.
Korin: Korin is a martial arts master who played a larger role in Dragonball, but is best known for growing the magical Senzu beans that NONE of the Z fighters could live without.
Nail: Nail was the strongest Namekian warrior living on the Namek and was Guru's guardian.  He bravely fought Freeza while Dende gave Gohan and Krillin the password to summon Porunga.  Even though he was still alive (or if he would have died he would have been wished back) he merged with Piccolo and gave him much more power.