Dick Grayson was the original Robin, the sidekick to Batman. In adulthood, he struck out on his own as Nightwing.
After graduating from college, Dick had a falling out with Batman over the tactics he used. He consequently left Gotham City and traveled the globe studying various martial arts and crime fighting techniques.
When he returned to Gotham City. He rejoined Batman's campaign as Nightwing, but his relationships with Batman and Batgirl became somewhat strained.
In spite of this, he continued to work semi-regularly with Batman's crime fighting team.