Krillin is often seen by fans as a kind of picked-on underdog, but he is still one of the most important characters in the Dragon Ball Z anime. As Goku’s best friend since the original Dragon Ball, Krillin is arguably the MVP among the human fighters, even though Tien Shinhan is probably stronger than him. His Japanese name is Kuririn, which you can actually see written on the hat he wears during the Namek Saga. I did not know that for a very long time.
To be honest, Krillin is not really a coward, although he does tend to be realistic when facing opponents. When he learns that enemies far stronger than Goku was when he fought Raditz are arriving in less than a year, he volunteers to fight without much hesitation. For the most part, he holds his own against Nappa for a decent time and even manages to put Vegeta’s life in his hands at one point, which is a great moment of irony.
He is also more than willing to go to Namek with Gohan, which fits his character, since it mirrors what he used to do back in the day with Goku. He comes close to death many times, including during his fight with Frieza. After the battle is almost over, Frieza lands a devastating attack that ironically triggers Goku’s transformation into a Super Saiyan.
Technically, Earth’s Dragon Balls cannot revive Krillin again because he had already been revived before, so they use Porunga instead to bring him back. Convenient, I know.
By the Cell Saga, however, his role as a fighter becomes mostly minor. He develops a crush on Android 18, which leads to one of the more surprising developments in the series. You could even assume this reflects the previous timeline Trunks came from, although in that timeline Krillin was killed instead of eventually marrying Android 18.
I always thought it was hilarious when I first watched it as a teenager that he ends up marrying an Android and even has a child with her. This confusion mostly comes from translation differences, since the original Japanese more often describes these characters as artificial humans rather than literal robots. Android 17 and Android 18 are cyborgs, meaning they were originally human before being modified into artificial beings.
Krillin does not do much against the villains during the Majin Buu Saga. He is turned to stone by Dabura, later restored, and mostly stays at Kami’s Lookout afterward. Like many other characters, he becomes a mortal victim again when Super Buu turns him into candy and eats him.
Along with Chiaotzu, Krillin is one of the characters who has been revived multiple times using the Dragon Balls. While he does have a somewhat more notable role in Dragon Ball Super, but in the post-time-skip portion of Dragon Ball Z, his time as a frontline warrior is largely over, much like many of the other human fighters.