Tien is one of the most notable characters in Dragon Ball Z, even though he doesn’t actually do much throughout the series. Technically, he’s one of the core members of the Z Fighters, but he’s definitely far less important than he was in the original Dragon Ball.
His most important saga is the Saiyan Saga, where he’s basically the first line of defense before Goku arrives. He does power up during the year they had to train, but obviously it doesn’t make much of a difference. He watches Chiaotzu die and even gets his arm sliced off by Nappa.
Since a lot of Americans, myself included, saw Dragon Ball Z before the original Dragon Ball aired properly, some of his moments didn’t make much sense at first. He uses the Tri-Beam technique with the one arm he has left and dies after firing it. But when he fights Goku in the original anime during their tournament battle, he explains that using the technique at full power can kill its user. He was more than willing to sacrifice himself if it meant killing Nappa, but it only barely singed him, which makes the moment even more embarrassing.
Like the other fallen Z Warriors, he ends up on King Kai’s planet and trains there. He even has that awkward moment where he struggles to tell a joke to pass King Kai’s test.
Chiaotzu and Tien eventually make it back to Earth after the Dragon Balls are used to wish them back. Still, it’s hard to imagine the two of them actually competing with Super Saiyans.
Surprisingly, though, Tien does take on Semi-Perfect Cell one-on-one. He actually manages to hold him off for a while using the Tri-Beam, which, oddly enough, doesn’t kill him anymore. It doesn’t really damage Cell much, but it’s enough to stall him and buy time for the others.
He tries a similar move against Super Buu with far less success. At one point, he’s even considered as a candidate to fuse with Goku using the Potara earrings, which honestly might have worked. But he ends up dying when Kid Buu blows up Earth, gets revived again, and after that he’s pretty much irrelevant.