Like most novel readers say, he's only misunderstood. He was portrayed as a villain in kmiyc and he was seen odd in kml. The author has been putting glimpses of him in other stories coz he is If You Can universe's main character. They wouldn't create him and Dane first and write Desire in 10 years if they are only going to make him as a simple "psycho and dumb blonde". He is a complex character that would make you think. I guess all readers who understood his character well ended up liking him and protective of him.
I also hated him in kmyic you see, but not to the point of closing my mind to hearing his story. I came to realize that a lot of context was missing in kmiyc manhwa adaptation and the novel clearly have incomplete information. These stories are connecting so I get author's intention though.

I don’t know I feel this overwhelming protectiveness of Grayson. I don’t know anymore like why in each story he’s been presented in as a background character. I always seem to find him. He’s like something that’s unforgettable and I feel like he’s the author‘s favorite as well is a psychopath wearing me down like is he seriously making him want me to protect him like a mother Bird or something?(⊙…⊙ )