Ryūsui

is a genin-level shinobi from Amegakure, though he has a scratch through the village symbol on his forehead protector to indicate his loyalty to Pain.

Background
He and Yūdachi were the ones delivering corpses to the highest tower in Amegakure, where Pain allegedly resides. Their most recent delivery was the corpse of what would be the second Animal Path.

Itachi Pursuit Arc
He and his partner, Yūdachi were captured by Jiraiya. When they refused to tell Jiraiya any information on Pain, even after tickle torture on his partner, he transformed Yūdachi into a frog, which frightened Ryūsui into divulging information about Pain. Jiraiya then tries to control Ryūsui in an attempt to find Pain, but is found by Konan. Ryūsui then follows Konan's orders to leave and is able to escape.

Trivia

 * The anime expands on his situation, seeing as he is captured by Jiraiya who learned about him from his wife, Tezuna.
 * means "running water", or "stream".