As a kid he was Goku's
rival in training. And at that time he wanted to be the best and strongest
just like Goku, infact at
one time Krillin was stronger than Goku. Overtime though Krillin and Goku became good friends.
Throughout the series there is one thing Krillin kept in the back of his
mind. He always wanted to get married (this is not
shown much at all the american version). Eventually, Krillin did get married to Andorid #18 after she was turned into a full
human by a wish from the dragon, Krillin and #18 even have a child together. While Krillin is a pretty important character in
DB and the first half of DBZ after that his significance as a character becomes pretty small, and even less by DBGT.