Yahiko

is a deceased ninja from Amegakure; his body acts as the Deva path of Nagato's Six paths of Pain.

Background
Yahiko, along with Nagato and Konan, were orphaned by one of Amegakure's many wars when they were children, forcing them to fend for themselves. The three eventually encountered Jiraiya, who looked after them and taught them basic ninjutsu. At one point, Nagato saved Yahiko after a Stone Chunin injured him, and kill the shinobi in a berserk rage. Afterwards, Yahiko vowed to become strong enough to protect Konan, Nagato, and eventually all of Amegakure.

Sometime after Jiraiya's departure from Amegakure, the three gained a reputation as a talented shinobi team. Later, Yahiko died under unknown circumstances. The trauma of his death presumably drove Nagato to pursue Yahiko's dreams, as well as adopt a harsher view of life.

Nagato, using his Rinnegan and chakra receivers, incorporated Yahiko's body into his Six paths of Pain jutsu. Since then, Yahiko's corpse, now the Deva path, has served as Nagato's main body while using the technique. This body is held in higher regard by Nagato and protected to a greater degree than the other Paths while in combat.

Personality
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 was his dream to stop the endless rain, which he compared to tears, and protect the nation's citizens.

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 the first to ask Jiraiya to teach them ninjutsu. He also seemed to act as an older brother for Nagato, reminding him to not cry and act more manly. He also wished to become stronger so as to protect Nagato and Konan.

Water Release
Yahiko had the ability to use water-based jutsu including Water Release: Violent Water Wave, in which he was able to shoot a jet of water from his mouth. His affinity for Water Release ninjutsu was presumably carried over by Nagato in the Deva path's ability to control rain.

Trivia

 * Yahiko apparently died around the age of 25-30 since a dead body cannot age.
 * Yahiko shares his birthday with his former teammate Konan.