Haru is a childhood friend of Yuki. He had been thought to have lost his life at the age of nine by falling into an underground cavern. Actually, Haru survived among a primitive race of mutant humanoids named "Hyoomans," who, like Finn, wear uniquely animal-themed hats. He was raised by them and learned the Hyooman culture, hiding his identity with a dog hat and going by the name "Rin-Tin." However, after Finn, Jake, and Yuki helped to save the Hyoomans' home, "Beautopia," Haru returned to Ooo with them.
Both Haru and Yuki have Japanese names: Haru's name means "spring" and Yuki's means "snow."