Aaron was the son of Grace O'Brien, who was proud of him whilst watching him grow up. (TV: Resolution) Around the age of nineteen, in late 1998 or in 1999, (TV: The Woman Who Fell to Earth) he had a son named Ryan Sinclair.
In 2012, Ryan's mother died from a heart attack, after which Aaron's relationship with his son soured to the point that he practically abandoned him. Ryan speculated that Aaron left because he reminded him of his mother. (TV: The Tsuranga Conundrum) With his father unable to cope in the wake of his mother's death, (TV: Resolution) Ryan ended up in Grace's care. (TV: The Woman Who Fell to Earth)
In 2017, Aaron visited Ryan, but the encounter ended poorly after Ryan "got mad" at Aaron for constantly missing out on his life. (TV: The Tsuranga Conundrum) Aaron failed to attend his mother's funeral in September 2018, (TV: The Woman Who Fell to Earth) both because he was ashamed and so that he would not have to accept she was really dead. (TV: Resolution)
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 his step-grandfather, Graham O'Brien. Ryan did not respond to the letter, resenting his father's definition of family. (TV: Arachnids in the UK)
Reconnecting with Ryan
On 1 January, 2019, Aaron arrived at Graham's house looking to make amends with Ryan for not being there for him, citing the New Year as an inspiration to start afresh. There, he met the Thirteenth Doctor and Yasmin Khan before going to a café with Ryan to talk. After failing to sell his microwave oven to the waiter, Aaron tried to convince his son that he had changed, asking Ryan what he could say to help. Ryan then lectured Aaron on how he had failed him as a parental figure. Later, Aaron returned Ryan to Graham only to stay to for a while as Ryan went away with the Doctor and Yaz in the TARDIS.
Whilst awaiting their return, Graham confronted Aaron about not coming to Grace's funeral and Aaron confessed his reasons for not attending. When the Doctor and the others returned, Aaron and Graham joined them in confronting a Dalek, Aaron taking apart his microwave oven to help destroy the Dalek's casing. However, the Dalek clasped on to Aaron and used him as a bartering chip to force the Doctor and the others to take him to Skaro.
Using Aaron as a host, the Dalek forced the Doctor's hand into bringing it into her TARDIS. However, the Doctor tricked the Dalek, sending it into a supernova. Fearing for his father's life, Ryan finally forgave him. He grabbed a hold of Aaron and managed to tug him in, saving his father's life. Once returned home, Aaron said farewell to his son, finally making amends. (TV: Resolution)
Two years later, Ryan told the Doctor that he had seen his dad during the ten months that she was gone. Ryan stated that Aaron was "at the oil rigs again" but was due back in a couple of weeks. However, they were doing well and reconnecting which Ryan was happy about. (TV: Revolution of the Daleks)