Aaron (Resolution)

Ryan Sinclair's father was the son of Grace O'Brien. (TV: The Woman Who Fell to Earth)

Upon seeing Yoss Inkl unexpectedly pregnant, Ryan recalled that his father was around the same age as Yoss and Ryan, who was nineteen, when Ryan 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)