The giver who had been released before Jonas was a girl. She had been selected 10 years before Jonas, but the girl did not have the qualities required to be successful. She was also a 12 and was carefully chosen for the honor of becoming the new giver. She could not handle the pain.
Her name was Rosemary. She was too weak emotionally to handle the emotionally painful memories and after only five weeks she asked for release. After that incident the Elders put in the rules that the Giver could not ask for release, but because it was not in the rules at that time the Elders had no choice but to grant her request. When she was released the entire community had to endure the memories she had received.
"In Chapter 20 The Giver tells Jonas, in a dramatic declaration, that Rosemary was his daughter. Whether he means this figuratively or literally is another matter readers will decide for themselves."