Katsuya develops parts of Takeo's insane personality, such as the obsession he has with the thought that Kayako has betrayed him by falling in love with Kobayashi, and his eyes change and appear more evil.
Katsuya contributes to Takeo's erratic behavior, including his fixation on Kayako's supposed betrayal by falling in love with Kobayashi and his eyes transforming into a more sinister appearance.