Rui is a small Chihuahua that lives near Tsuna's house.
It seems to like everyone but Tsuna, who, unsurprisingly, is afraid of Rui. Rui's first appearance was on the day that Tsuna met Reborn. While Tsuna was going to school, he accidentally stepped on Rui's tail and Rui got angry. Its leash wasn't attached to anything and it tried to attack Tsuna. He was saved by Reborn, who easily calmed Rui down.
Before the Kokuyo Arc, Futa hid in Rui's doghouse while he was being chased by Todd Famiglia. According to Futa, Rui's doghouse is ranked 22nd out of 305 hiding spots in Namimori. Rui was also shown during Iemitsu's flashback, when Timoteo first met Tsuna in Japan. While Tsuna was playing outside, Rui came to play, but Tsuna was so scared that he started crying, scaring Rui away. The owners of Rui also have a bigger and scarier dog that lives with Rui. They seem to get along, as he tried to attack Tsuna when he refused to give food to Rui.