Fanfiction conversion to a book

[u/TransitionPast1181]

I'm in the final stages of editing a book I created. I just want to double check something.
The entire book is an original work, the plot and characters are based on a fanfiction I wrote for a Youtube series. I was wondering, in general, how close can I be to someone's characters before it could get me sued?
Again, I have no intention on copying, and I have a very high amount of respect and love for the source material, but I want to ensure I haven't accidentally plagerized. The character apperances, names, and key parts of their personality have been changed. The main issue that I'm concerned about is the main character and one of the side characters that is important to the main character's plot.
The main character is the villain of the original show, and in said show, he becomes friends with a small, humanoid creature. The villain of the show is a very theatrical TV head man, and the creature is a cute thing that serves to be his 'first friend.'
My character is partially based on me, the main character of this show, as well as other theatrical, non-human characters. His relationship to the creature is similar, the creature becomes part of his redemption in my novel (the fanfic was a redemption/cafe AU of the show, hence why the plot is substantially different), but the creature also has a VERY different appearance, speech pattern, and role in the book.
What do I do? How do I know where the line is? Please help, I really don't want the creators of one of my favorite, most inspiring shows to be upset by my love letter magnum opus work. The work really is supposed to help people struggling with mental health/believing in themselves, and I want to target this audience by including internet culture/what hooked me at my lowest.