Hideyoshi Sarutobi, often referred to as Saru, is a friend of Nagasumi who is often portrayed as a perverted monkey. He later became Kai's "servant" and always protects and helps him when he is in need (although reasons are more likely because of his fortune). Saru occasionally appears dressed as an old hermit, "ero-sennin" (erotic/perverted thousand-year-old), who periodically appears to offer insights of perverted sagacity. (Source: Wikipedia)
