Mildred

Mildred affects montag in many ways...

During the beginning, she scares the $#!t out of montag, because she overdosed on sleeping pills.

In the middle, she reports montag to the firemen.

And at the end, She was killed in the nuclear attack, and maybe montag felt remorse.

I think she affected him by being his reality into the life he didn't realize he was living.