Something that is "supple" is soft and bendable. When the word is applied to people, it often means someone who is good at groveling and brown-nosing. And as with Mr. Thwackum (thwack'em), Fielding clearly chose this character's name for a reason. He is a friend of Squire Western's. He is also a clergyman who serves on Squire Western's lands, which means that Squire Western pays Mr. Supple's salary.
Mr. Supple travels with Squire Western on his search for Sophia. And whenever Squire Western seems like he is going to get too violent with his daughter, Mr. Supple tries to step in and hold him back. When Squire Western finds Sophia in London at last, he looks like he's going to hit her. Mr. Supple adds: "I entreat you, sir, to be a little more moderate […] you frighten the young lady so, that you deprive her of all power of utterance" (15.5.10).
Now, we think it's great that Mr. Supple is trying to calm Squire Western down and prevent him from frightening Sophia. We also know that he is supposed to be a good guy and generally a voice of reason. But his efforts to stop Squire Western appear a little, well, lame. "A little more moderate"?! Squire Western almost hits his daughter! He's got to do a lot better than "moderate" if he wants to redeem himself in our eyes.
Mr. Supple helps Sophia a little bit—and he keeps her relationship from her father from getting physically abusive—but he is too soft and yielding to be a real support to her. He wants to stay in Squire Western's good graces, since the squire pays his bills. But still, we agree with Mrs. Honour when she wishes that he "had but a little more spirit, to tell the squire of his wickedness in endeavouring to force his daughter contrary to her liking" (15.7.2).