Lord Fellamar is an aristocrat (duh: lord) and a member of the British army. He is also an oddly contradictory character: on the one hand, as an army man and as a nobleman, he is very serious about honor and the maintenance of his good name. When Squire Western responds to his proposal for Sophia's hand in marriage by saying, "You are a son of a b— […] for all your laced coat" (15.5.26), Lord Fellamar storms off and, later, through his captain-assistant-person, almost challenges Squire Western to a duel. Clearly, Lord Fellamar is used to a certain kind of social code, within which he feels secure and able to behave properly.
But on the other hand, Lord Fellamar is also embarrassingly easy to influence. When he confesses his sudden passion for Sophia (whom he meets in a crowded theater when a fight breaks out in the audience), he quickly agrees to Lady Bellaston's terrible plan that he should rape Sophia first and then marry her.
He believes Lady Bellaston when she says that Sophia is hopelessly in love with a useless rascal named Tom Jones, and that forcing her into marriage would actually be rescuing her from the dark fate of seduction by Tom. Lord Fellamar decides to go ahead with this vicious and deeply dishonorable plan, even though he feels deep inside that it probably isn't the right thing to do.
Luckily, Squire Western interrupts Lord Fellamar before he actually succeeds in assaulting Sophia. But his feelings for her don't go away. And Lady Bellaston continues to play on them, encouraging Lord Fellamar to send a gang of guys after Tom to kidnap him and put him on a naval ship. Again, Lord Flamer's scheme doesn't succeed, but the gang does get Tom thrown in jail after his duel with Mr. Fitzpatrick.
Why does Lord Fellamar follow Lady Bellaston's advice so thoughtlessly when his own instincts warn him that he is not doing the honorable thing? For one thing, Lord Fellamar trusts Lady Bellaston's opinion about how to seduce women because she is also a woman. But while Lady Bellaston may be a woman, she is also a bad woman—obviously, her advice is biased and untrustworthy.
Lord Fellamar kind of redeems himself by the end of the book. When he realizes that Tom is actually a gentleman and not a scoundrel, he immediately gives up all hope of marrying Sophia. He also gets a friend to help him testify in court that Mr. Fitzpatrick is not dead, so Tom cannot have murdered him. By getting Tom out of jail, it appears as though Lord Fellamar has made up for his earlier lousy behavior: he has resolved to "do everything in his power to make satisfaction to a gentleman [Tom] whom he had so grossly injured" (18.11.6).
While we appreciate Lord Flamer's efforts to make things better for Tom, we still have major issues with the guy. The problem is that he has no real moral instincts. He does his best to behave according to a code of honor, but he is also willing to set aside that code if that will get him what he wants. Like Lady Bellaston and all the other members of London high society whom we meet in this book, Lord Fellamar seems to care more about appearing decent than actually being decent. We can't exactly trust a guy like that.