Aaron (Resolution)

Ryan Sinclair's father was the son of Grace O'Brien.

As Ryan explained to the Thirteenth Doctor, his father was notoriously unreliable. He failed to attend his mother's funeral. (TV: The Woman Who Fell to Earth)

After Grace's funeral, he sent a letter to Ryan apologising for not being there. He expressed a desire to reconnect with his son and offered to let Ryan live with him as he was "proper" family, unlike Grace's husband Graham. Ryan told Graham that he disliked his father's notion of "proper" family. (TV: Arachnids in the UK)