The marten is the main antagonist of Kai No San Kyōdai.
The pine marten lived in the mountains of Kōfu in Yamanashi. After the less than 3-months-old Kai brothers were abandoned into the wilderness, the marten soon noticed them.
The marten had been stalking them and waited for the puppies to fall asleep in a cardboard box till dawn. When the Kai Brothers tried to sleep in peace, the marten attacked them. The marten's teeth sunk into Kurotora's right eye and when it was about to eat the puppy, Ben the Great Dane came just in time and drove the pine marten away, saving Kurotora.
Later on, when the puppies were beginning to hunt as they were taught by Ben, the marten watched them from the treetop and waited until nightfall. When the Kai brothers fell asleep, it attacked them again - this time it snatched Chūtora by his right ear and dragged him off. However, Chūtora managed to twist himself free from its jaws and fought against the marten together with Akatora and Kurotora. The fight was long and hard that the marten finally dropped dead from exhaustion and blood loss.
A flashback scene explains how the marten attacked the Kai puppies, slashed out Kurotora's right eye and was driven away by Ben.
- Some translations of the Ginga: Nagareboshi Gin anime mistakenly call him a dog.
- Early translation called the marten Tin, a miss translation of the japanese word for marten: ten
- Though he was white in the original GNG manga and anime, he is tan with brown markings in KNSK.
- Because of the style change, the marten looks oddly more doglike.