The final past Avatar Aang truly met was the previous Waterbending Avatar, a man named Kuruk. Unlike the other Avatars, Kuruk took a more laid back approach to being the Avatar. The world was at peace when he came into it, and so he let the people of the world solve their own problems. This led to him wandering the world and doing things he wanted to do, like dueling other people, and impressing women.
Eventually, he met a young woman from the Southern Water Tribe named Ummi, and instantly fell for her. The two were to be married, but on the night of the ceremony she was pulled into the Spirit World, and taken by Koh, the face stealer, who promptly took her face for Kuruk arrogance.
Kuruk vowed to get her back, and every year on their annivesary would go and fight Koh. Eventually, he would be at the point where he could kill him, but Koh revealed he had Ummi's face, and that if he killed him, she would die too.
Kuruk left a shell of his former self, constantly living in regret over what he did. He advised Aang to "shape your life, and the world" as it was his failing to do that that cost him Ummi.