How to Create a True Foil Character

A true foil has to be missing something that the character they're foiling is then offering to them. We can see much more clearly what a character believes when we see that in the POV of another character who is disagreeing with them. It also makes it an external conflict rather than an internal conflict, which is inherently more interesting. Internal conflicts are great, but the longer you go on them, the more we get into navel gazing and wallowing in our problems.