Aaron (Resolution)

Ryan Sinclair's father was the son of Grace O'Brien. (TV: The Woman Who Fell to Earth) He was "around" nineteen-years-old when his son, Ryan Sinclair, was born. (TV: The Tsuranga Conundrum)

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)

Ryan theorised that his father avoided him because he reminded him of Ryan's mother, whom he had loved. (TV: The Tsuranga Conundrum)