Prior to the series beginning Paifu was born to two parents, a werekoala father and a female vampire, when he was younger Paifu's had left the village to get supplies only to go on a rampage while in his werekoala form. This angered the humans and had been killed.
Paifu is a vampire-werekoala hybrid who is friends with a ghost named Jose both live in a village in dark forest called Batwing Ridge where they attend night classes with their fellow monsters and humans who live in a nearby village. Paifu is known be mischievous and often plays around. One day while playing outside Paifu and Jose decide to go to the house of a human rumored to be a murderer. Paifu notices what appears to be an orange spirit out in the sea and mistakes it for their friend. They arrive at the light source only to find that it was a torch on a raft belonging to the world-renowned sumo wrestler Maruyama, whom they feared. In spite of becoming acquainted, Maruyama teases Paifu and picks up two sticks producing a cross gesture to scare him off believing he, like some vampires in stories, fear a cross. On the contrary, this action forces Paifu to transform into a rampaging were-koala. Jose later explains that unlike most vampires Paifu is also a were-koala and that the only way to change him back is with a round object and Jose demonstrates by turning into a round ball. Once back to normal Paifu begins learns more about Maruyama's past and learns that the rumors about him were true. The next day Paifu and Jose learns about a disease that only affects monsters is spreading throughout Batwing Ridge. While on his way to his night classes he encounters his classmate and rival Apron who challenges him to fight to decide who is the strongest. After an exchange between the two, Paifu finds himself dragged into a fight and is late for class. A day later Paifu's mother later tells him that school has been canceled for a while due to the serious flu. He celebrates by going to José's home and runs into the village doctor. He finds out from the doctor that the flu going throughout village is a virus called the Monster Flu (お化け風邪 Obake Kaze?), a disease that affects only monsters and kills the victim in a month's time. When he reaches José's house he sees Jose's ill father, the doctor's suspicions are confirmed to everyone there and reveals that the cure to the Monster Flu can be created by a witch that resides at the top of Horned-Owl MountainThe doctor also tells of a forest that one must cross and that a terrible monster dwells along the forest path which leads to the mountain. Due to the adult monsters being sick, Paifu and José volunteer to make the trip, to help on their journe Paifu tries to convince the former sumo wrestler Maruyama to occompany him, however he demands are 1,000,000 yen for his troubles. However Paifu deceives him by claiming that the villagers would pay him. As Paifu and his friends, Arpon joins the team, exclaiming that he wants to be a hero for the villagers. While journeying along road Paifu learns more about the outside world and encounter cruel humans. Eventually Paifu and his friends reach a forest area located by Horned-Owl Mountain. As they near it, Arpon suddenly falls ill by the Monster Flu. They come to a nearby family for aid; in the midst of their troubles, Paifu and Maruyama save them from being attacked by a gang. In exchange, the family cares for the sick Arpon while Paifu, José and Maruyama take off for the forest. Before they leave, they are told by the family of the forest monster's supposed weakness, whistling.
Upon entering the forest, Maruyama has José whistle since he is apparently the only one who can, while Paifu practices his whistling since José will eventually get exhausted. José, instead of getting fatigued from whistling, suddenly falls ill to the Monster Flu as well. Their fear of the monster finding them becomes a reality and they are attacked by the forest monster Baroaba. Maruyama holds his own against the behemoth and tells Paifu to start learning how to whistle immediately before they are killed. Although several attempts had been unsuccessful, Paifu finally prospered in whistling and Baroaba is literally downsized by the sound, proving the weakness true, and is pounded by Maruyama. Before Maruyama finishes him off, he discovers that Baroaba only attacks to protect the forest's rare wildlife; Baroaba apologizes for the misunderstanding and decides to help them get to their destination, the witch's house.
With help from Baroaba who stays to care for the ill José, Paifu and Maruyama finally reach the summit of the mountain. They then encounter the witch's servant, an oni named Leonardo. After solving a riddle, they are allowed to meet the witch and fortunately retrieved the influenza medicine. On the way down the mountain, Maruyama accidentally slips and falls off. Paifu makes a daring rescue by overcoming his inability to fly by learning flight at the last minute and saves Maruyama and himself from immediate death.On the way back, Maruyama talks of how he is going to buy a boat with the money he makes forcing Paifu to reveal that he had lied to him about the payment of 1,000,000 yen. Maruyama admits that he is disappointed, but not infuriated as Paifu saved his life. The four are praised as they returned to the village. One month later, Paifu, Jose and everyone else from the village meet Maruyama out at sea with a ghost boat that all fixed up together.
He also has some skills as martial artist as he is able to take on his rival and fellow monster Airpan along with some human bandits who attempted to mug him and his friends.When he observes any cross-like figure for more than three seconds, Paifu shapeshifts into a crazed and powerful were-koala and can only change back to his normal state after looking at a round object. While being a were koala he has tremendous a Despite being part vampire, he can not fly because he skipped flying practice. Though later on he is able to overcome his personal inability to fly as he was able glide and carry Maruyama down a mountain.