User blog comment:Zeref Fullbuster/Dabi is Shoto's eldest brother Toya/@comment-43974514-20191004021033/@comment-24332696-20191015013558

He’s changed, and we will learn how and what changed him to become the way he was. A theory I just thought of was jealousy and it seems to line up with how he mocked him when he failed to rescue Bakugo. He mocked him like someone who was considered perfect that failed.