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Aaron was the son of Grace O’Brien, and the father of Ryan Sinclair. He worked as an oil rig engineer.
He was “around” nineteen years old when his son, Ryan Sinclair, was born (The Tsuranga Conundrum) in 1999 or late 1998. (The Woman Who Fell to Earth)
In 2012, Ryan’s mother died from a heart attack. Afterwards, Aaron’s relationship with Ryan soured, to the point that he practically abandoned his son. Ryan theorised that Aaron avoided him because he reminded Aaron of his mother, whom Aaron had loved. (The Tsuranga Conundrum) With his father unable to cope in the wake of his mother’s death, (Resolution) Ryan ended up in Grace’s care. (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. (The Tsuranga Conundrum)
Aaron failed to attend his mother’s funeral in September 2018, (The Woman Who Fell to Earth) both because he was ashamed, and so that he would not have to accept she was really dead. (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. (Arachnids in the UK)
On 1 January 2019, Aaron arrived at Graham’s home, where he met Yaz and the Doctor. He desired to make amends with Ryan for not being there, citing the New Year as an inspiration for him to start fresh. He was surprised to find Ryan called Graham “gramps” but left with Ryan to a café all the same. After failing to sell his microwave oven to the waiter, Aaron tried to convince his son 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 Yasmin in the TARDIS.
While 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.
While 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 his father. He grabbed 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. (Resolution)