Character Analysis
Frederick Fairlie is definitely a horrible person. We can't decide if he's hilariously awful or if he's just awful-awful: he's hypochondriac and an egomaniac. He's pretentious and narrow-minded. He treats others with contempt. He's rude and almost willfully stupid.
If we had to pick one overriding personality trait for Frederick Fairlie, though, it would be selfishness. It leads him to treat others badly, causes him to not listen, and results in him making seriously bad errors in judgment.
Here's Fairlie in action:
My idea is that she began by telling me (through Louis) that her master had dismissed her from her mistress's service. (Observe, throughout, the strange irrelevancy of the Young Person. Was it my fault that she had lost her place?) On her dismissal, she had gone to the inn to sleep. (I don't keep the inn—why mention it to me?) (2.2.1.30)
Wow, Mr. Fairlie. Way to make the Grinch look like a big-hearted guy.
Mr. Fairlie is so obnoxious that his narrative is almost painful to read. Here we see his selfish disdain in action. It's this selfishness that leads him to do what Fosco wants and to later toss his niece out of the house because he can't be "bothered" with all the drama. Listening to Marian's explanation of the Anne/Laura switch was too mentally taxing, apparently.
Fairlie might seem like a ridiculous character (and he is), but he's also a dangerous one. His disregard for his niece and her sister puts them both in serious danger. Mr. Fairlie's routine of burying his head in the sand is not a good way to live, and the narrative judges him harshly for it.
In the end, Frederick Fairlie conveniently kicks the bucket, paving the way for Limmeridge House to come under new and better ownership when the Hartright family (plus Marian) move in.