Who’s the saddest character ?

For me it’s a close run between Sirius and Ariana.

Ariana : got brutally assaulted as a kid and so severely traumatized she never recovered. She has to live a life of pain, shut up at home, while her favorite brother is away at school most of the year. Then she accidentally loses her Mom and main caregiver, and suffers an early death herself.

Sirius : brought up by an awful and unloving family, kicked out at 16. Then it is safe to assume he may have had a few happy years had he not been involved in the first wizarding war. Then at 21 his best friend is brutally murdered, as a consequence of having persuaded him of a change of plan, so on top of the pain he had to suffer the guilt. He is unjustly accused of the murder and goes away 12 years in Azkaban without a trial. From this day on the entire wizarding world thinks he is a terrible criminal. Finally after he escapes he has to live in hiding, disguised as a dog, living in caves and eating rats. He fails in avenging his best friend’s death. He has to live in hiding still until Voldemort returns when he has to start living locked up in his childhood home that he thoroughly dislikes. Finally he dies still a young man, murdered by his cousin.