Basil is the artist who paints the beautiful portrait of the twenty-year-old Dorian Gray. Basil tells Lord Henry that he likes Dorian "immensely" and that Dorian has cast an enchantment over him. Dorian's harmonious presence has changed Basil. Dorian has become Basil's muse or artistic inspiration:
His personality has suggested to me an entirely new manner in art, an entirely new mode of style. I see things differently, I think of them differently.
Naturally, Basil does not want the decadent Lord Henry to meet Dorian, and just as naturally, Lord Henry's curiosity has been greatly excited. The two meet, and Dorian makes his fateful pact with devil so that his picture ages instead of his body.
When Dorian's cruelty causes Sibyl to commit suicide (followed by more insensitivity from Dorian), Basil speaks with Dorian. Basil tells him he is wrong in his attitude toward Sibyl and blames Lord Henry for corrupting him. All of this angers Dorian, who refuses to let Basil see the portrait he painted, which has begun to look cruel in response to the degradation of Dorian's character. Basil functions in this episode as Dorian's conscience, which means he is largely banished from Dorian's life: Dorian does not want to face his conscience.
The two don't see each other for many years until they happen to meet on the street when Dorian is thirty-eight. Basil, still functioning as Dorian's conscience, accepts Dorian's invitation to a visit and then talks to him about all the people he is rumored to have ruined. Basil says he would like to see past his former friend's beautiful appearance and into Dorian's soul. Dorian shows him the picture, hidden in an attic, which reveals his now hideous soul. Basil reacts with horror, and Dorian stabs and kills him. Dorian is symbolically trying to kill his troubled conscience, but this fails: After Basil's death, Dorian is more and more filled with anguish and regret over his empty and misspent life. He finally stabs the portrait, killing himself.
Basil is an exemplary character with a balanced moral compass who tries to help Dorian. Because of all this, his relationship with Dorian deteriorates. Dorian doesn't want to be confronted by Basil with his wrongdoing. However, in the end, Dorian cannot kill the moral voice Basil represents or escape facing what he, Dorian, has become.