Complex sentence: When she was born, her parents let her have a soul so she could socialize in the human world, which makes her sweet and down-to-earth, as opposed to most vampires, who are usually heartless and cunning.
Split into simpler sentences: When she was born, her parents let her has a soul so she could socialize in the human world. A reason which also explains her sweet and down-to-earth nature, as opposed to most vampires, who are usually heartless and cunning.