The Winchester Sister
Dean and Sam Winchester have a younger sister, Ashlee, and by younger, I mean she is 14 years old. Told you, younger! Anyway, the guys discovered they had a sister after their father, John, died. She called them, crying, about how her mum was murdered by a human with black eyes. Naturally, Dean and Sam were skeptical about Ashlee and her story but when they met her, she looked a lot like Sam and Dean. (Photo beside text; imagine she looks like a 14 year old and has Dean's eyes). They did a follow up of her background and her birth certificate and in the Father section it says, 'John Winchester'. While they were there, they banished the demon to hell. She started off on her journey with them.
~Startingoff at the start of Season 4 when Dean returns~