Boy (Heaven Sent)

A 7-year-old Gallifreyan boy was walking through Gallifrey's Drylands when the Twelfth Doctor escaped from his confession dial.

The Doctor told him: "Go to the city. Find somebody important. Tell them I’m back. Tell them, I know what they did, and I’m on my way. And if they ask you who I am, tell them I came the long way round." (TV: Heaven Sent, PROSE: A Brief History of Time Lords)

The Doctor then went to the boy's home, the barn from the Doctor's own childhood, and ate soup while the boy and other Gallifreyans watched. Rassilon then arrived to threaten the Doctor. (TV: Hell Bent, PROSE: A Brief History of Time Lords)

When the boy turned 8 he was, like every Gallifreyan child, taken to the Untempered Schism. Passing the evaluation, the boy was taken to become a Time Lord. As he grew up, the boy became somewhat disillusioned and cynical towards his society, even holding some bitterness towards the Doctor, once his boyhood hero, for never taking the chance to bring true peace to Gallifrey. When the boy was older, he authored a textbook on Time Lord history for the Academy, annotating it with several theories he had on unexplained aspects of his race's history. (PROSE: A Brief History of Time Lords)

Behind the scenes
Although identifying the Barn as the boy's current home as well as the Doctor's former one, A Brief History of Time Lords fails to explicitly identify the boy as a relative of the Doctor or member of the House of Lungbarrow.