Sibyl Vane is too good for her own… good. She's sublimely beautiful, amazingly talented, and totally innocent—which makes her a magnet for Dorian:
"I love Sibyl Vane. I want to place her on a pedestal of gold and to see the world worship the woman who is mine. What is marriage? An irrevocable vow. You mock at it for that. Ah! don't mock. It is an irrevocable vow that I want to take. Her trust makes me faithful, her belief makes me good. When I am with her, I regret all that you have taught me. I become different from what you have known me to be. I am changed, and the mere touch of Sibyl Vane's hand makes me forget you and all your wrong, fascinating, poisonous, delightful theories." (6.8)
She's everything he sees in himself, only better, since her purity is totally unspoiled. However, her lack of cynicism makes her particularly vulnerable to just about everything; as she herself says, her only experience up to this point is a kind of sheltered half-life on the stage:
"Dorian, Dorian," she cried, "before I knew you, acting was the one reality of my life. It was only in the theatre that I lived. [...] You came -- oh, my beautiful love! -- and you freed my soul from prison. You taught me what reality really is. [...] You had made me understand what love really is. My love! My love! Prince Charming! Prince of life! I have grown sick of shadows. You are more to me than all art can ever be. What have I to do with the puppets of a play?" (7.15)
Once real life intervenes, it proves to be too much for poor Sibyl to handle. Her suicide is the great tragedy of this novel; it's the turning point for Dorian, and it's the death of the one truly innocent figure we encounter.