sorry if it's vague but it's been bugging me for a while now.
The character watched this cartoon as a kid. His brother died when they were children due to some mistake or accident on his part, and he carries guilt over it. I think he puts the responsibility on the cartoon character or a hallucinated someone with a similar name as a trauma response. The story takes place when he's grown up, and at some point he or the audience discovers the truth about what really happened with his brother's death. The show/movie itself is not animated — the cartoon only exists within the story's universe. The brother dies I think when they go playing in some cave or somewhere outside. The cartoon character may be some guy named Dr something iirc the show was kind of those edutainment cartoons I think. Again, I'm super fuzzy on the details so sorry.