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, which caused him to unlock his Rinnegan and kill the shinobi in a berserk rage.

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 technique. Since then, Yahiko, 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.

Yahiko had a strong "eye for an eye" philosophy, and 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 dream is to stop the endless rain, which he compares to tears, and protect the nation's citizens. He had the ability to use water-based jutsu, which was carried over by Nagato in the ability to control rain.

Abilities
The water jutsu he is able to use seems to shoot a jet of water from Yahiko's mouth.

Trivia

 * Yahiko apparently died around the age of 25-30 since a dead body cannot age.