Nyen doesn't like Sebastian (the human pet you gave your younger brother, Randal, as a birthday present), and from the beginning he made it clear that he is the most important pet in the house. Well, there was even a "graph" with the position of the others, with you being the "1" of the home.

You also have other cat, Nyon, but Nyen is the most aggressive of the two. For example, he has directly threatened Sebastian (whom he considers an inferior being) repeatedly, directly or indirectly.