Alongside his fellow war orphans, Nagato and Konan, he founded and led the Akatsuki in an attempt to bring peace. Following Yahiko's death, Nagato would turn his body into the Deva Path of his Six Paths of Pain, which he used in his public image as Akatsuki's new leader.
Yahiko's personality was similar to Naruto's. His dream was to be a leader of his village, and he never gave up no matter what. When he was alive, Yahiko had a strong "eye for an eye" philosophy. He hated the way Amegakure was looked at by the rest of the world, and hated what it had become. He was defensive of the village, saying that it was his dream to stop the endless rain, which he compared to tears, and protect the nation's citizens. Yahiko wanted to become the ruler of the world so there wouldn't be any more wars. Many of these traits were adopted by Nagato after Yahiko died.
Among the three Ame Orphans, Yahiko seemed to act as the leader of the group. It was Yahiko who first approached the Sannin to ask for food, and was the first to ask Jiraiya to teach them ninjutsu. With Jiraiya's influence on them Yahiko even seemingly started to adopt some of Jiraiya's mannerisms such as his boastful facial expressions. He also seemed to act as an somewhat tough older brother for Nagato and Konan, reminding Nagato to not cry as it won't do anything and to act more manly. He also wished to become stronger so as to protect Nagato and Konan. He had faith in Nagato, when he got older. Yahiko was willing to commit suicide rather than endanger Konan's life, displaying a very high level of loyalty to those he cared for. He also had romantic feelings for Konan which were reciprocated.
Yahiko was shown to be sharp-witted, having figured out "Madara" was an impostor and knowing his plan was just a ruse, and instructed Nagato and Konan to stay away from him. [Source: wikia]
You can download this Naruto paper model here: Naruto - Yahiko Deva Path Free Papercraft Download