Post History
Ok, considering the information in the comments, what you know for your character is that in the end he sympathizes the antagonist. I would take this as inspiration for his flaw. So in the end it t...
Answer
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/24575 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Ok, considering the information in the comments, what you know for your character is that in the end he sympathizes the antagonist. I would take this as inspiration for his flaw. So in the end it turns out the antagonist was right in a way. I'd make the reason why the protagonist didn't see that at the beginning (that the antagonist was somehow right) as the protagonist's flaw and prove it throughout the plot. For example the protagonist was too superficial or he was too conservative or the detective was too brutal with the criminals etc. The antagonist will "help" the protagonist grow this flaw of his. P.S. I always try to answer the WHY question and not the WHAT. If you have the what... happens for example, show the reader why actually it happens.