Prosopagnosia- he can't recognize faces (Jack). America's fattest teen had to have her house cut open so she could be transported to the hospital (Libby). And the two shall meet. Of course. So, Jack hasn't told anyone about his prosopagnosia. In fact, he figured it out through his journals. So, he tries really hard to look for identifiers such as unique hair, ears sticking out, etc. But, he also has some home life issues. Libby was always chubby, but when her mother died, she coped with lots and lots of eating. When she became America's fattest teen she was interviewed on TV and was then given a nutritionist and a counselor to help her deal with her anxiety and depression. Ok, so far, so good.
Here's where the book went downhill. First of all, Libby lost 300 pounds (hooray) but is still probably about 300 pounds. And she is proud of it. Cool. Students still moo at her, and there is a game that involves a cruelty to overweight girls that she has been a recipient of. And then, towards the end of the book, she is surrounded by boys wanting to kiss her, who have always had a crush on her. What? I was always overweight in high school, and I can't say there were boys lined up for me. Maybe it's jealousy, or maybe it's just not realistic.
Jack is also part of the cruel game. He and Libby got in trouble for it and had group counseling together, hence the friendship begins. He's mean. Then he's nice. Then he's mean. Then he's tormented for knowing he's mean which then makes him nice. But he likes Libby. A lot. That's cool, too. But when two extremes of the high schoolers meet, the whole story seems contrived. All the Bright Places, was IMHO, was far better.