The relationship between Tom Jones and Sophia Western in Tom Jones isn't a very complex one, though it is an important one and central to the story. The reason it isn't complex is that psychological elements are not emphasized and their association is straightforward. Tom's and Sophia's fathers' estates neighbor each other and so Tom and Sophia have been friends since childhood. Tom loves Sophia, and she loves Tom.
The wrinkle comes into play because Tom, though possessed of a good and generous heart that values others above himself, is an adventurer and not morally opposed to immoral adventures with willing ladies. Sophia finds this trait and activity less than appealing and so tries to keep Tom at arms length.
When Sophia runs away from home to escape being forced to marry the despicable and underhanded Mater Blifil, circumstances conspire for Sophia and Tom to be brought back together. Events show her that Tom has learned the lessons Squire Allworthy has tried all through the years to teach him and, once all circumstances have been resolved in Tom's favor, she consents to marry him.
Tom for his part has always felt that he can never be happy without Sophia, she is all his heart thinks of, but when fate lays opportunities at his energetic and attractive feet, he takes advantage of them. Tom shows that he is intrinsically worthy of Sophia and that he will ultimately partake of the virtue of her namesake because, though good at getting into trouble, he endangers himself to save and help others who are in need and of whom higher authorities are dismissive. The meaning of the name Sophia, derived from a Greek word, is "wisdom." Tom's innate qualities and his steadfast love of Sophia show that he will in the end be united with Sophia in person and in name.