1866 — Barnabas is born, the last child and second son to the Jameshill family. By the time he is born, his eldest sibling has already gone off to Hogwarts.
1869 — At three years old, Barnabas (who can rarely pronounce his own name correctly) is unaware of the family’s increasing good fortunes as he is more concerned with play time. He does, however, manage to exhibit some magical ability for the first time when he keeps relighting the lamp in his room. The nurse spends much of the night extinguishing the small flame only for him to relight it again out of a fear of darkness.
1870 — When Lupus is home, Barnabas has taken to following him unless prevented. He particularly enjoys grabbing the back of his brother’s clothing and encouraging him much like a coachman would his horses. Neighs and ‘clip clop’ noises usually follow.
1872 — Just as he loses his sister to Hogwarts, he gains his brother! Around this time, Barnabas decides that he doesn’t want to be called Barnabas anymore because he doesn’t think it sounds as good as ‘Lupus’ and he desperately wants to earn his brother’s respect and affection. #heroworshippingintensifies. Theodoric, his middle name, is what he chooses to go by instead. His middle name has the advantage of both novelty and familiarity. He stubbornly ignores all attempts at calling him Barnabas thenceforth and after several stubborn months, ‘Theo’ attaches itself as a nickname.
1873 — Ignorance is bliss. Theo is wholly unaware of Lupus’ accident, his main recollection of the time was the sudden move to the country. Generally liking the outdoors already, this move definitely pleases him. What doesn’t please him, however, is the change he senses in his beloved brother. While he had no idea what was going on in his brother’s life, he could sense the tension that had infiltrated the family. One night, unable to sleep, he snuck out of his room with the intention of seeking out his brother but upon entering his room, found it empty. Dismayed and curious, he stealthily wandered the house in search of his absent brother and by chance discovered him chained up. Before his very eyes, he witnessed the unexpected transformation of his brother into a wolf! Although wise enough to remain at a safe distance, he lacked the appropriate fear one ought to have of werewolves and soon learned how to predict when his brother would go turning into a wolf so he could try and watch. His later attempts were usually foiled and the punishment was always severe, not that that deterred him. During this time, he developed a fondness for wolf-based games which was probably very insensitive of him.
1874 — Being more in awe of his brother than ever before (as well as fascinated by the werewolf thing) it isn’t long before he blurts out the secret to Amity. He is punished thoroughly enough that he never makes the mistake of letting his brother’s secret out again. Lupus becomes a fully qualified Auror and gives Theo yet another reason to idolize him.

1877 — Theo’s idolization of his brother has thankfully subdued to deep admiration rather than full on hero worshipping. He is quite excited to go off to Hogwarts in September and finds himself in Gryffindor which pleases him to no end. He enjoys his classes but goofs off frequently. His homework is frequently late and often scruffy.
1879 — Theo picks up Care of Magical Creatures, Divination, and Earth Magic. The latter is chosen instead of Ghoul Studies so his brother doesn’t think he’s imitating him. 1881 — During his fifth year, Theo’s world is shaken to the core when he finds out Lupus has died and the details are so vague that it seems to make it all the worse. He starts acting out - quite badly too - until his drop so severely that a lecture about his career options could be affected with OWLs coming up. He decides to become an Auror like Lupus and soon develops the academic discipline he never bothered with before. His efforts are just enough to turn around his OWL prospects. His general behavior is still poor, however.
1882 — Amity drops the bombshell that Lupus isn’t actually dead. Theo is momentarily thrilled until he realizes that this doesn’t bring his brother back into his life, but the hope of seeing his brother again does much to lift his spirits. Feeling like his parents aren’t doing enough to locate his brother because obviously they would have found him already if they were, he takes matters into his own hands. He writes many letters to his brother but never receives any response. This, initially, does not dim his hopes.
1883 — In his sixth year, he has gotten into the habit of writing to his brother weekly until the summer exams arrive. He neglects writing for a few weeks to focus on studying and when he sits down to compose his first letter in a month, the hopelessness hits him. His brother is either truly dead or just doesn’t want anything to do with him or the rest of the family anymore. While not a bitter person by nature, he can’t help but feel resentment towards his missing brother. This bitterness helps him move on. Resentful or not, however, his love for his brother is unchanged as he doesn’t give up his ambition to be an Auror.

1884 — Theo graduates from Hogwarts with the necessary NEWTs to enter the Auror program and that is exactly what he does. His unruly behavior has subsided for the most part which makes all the difference in being allowed onto the program. Not long after, the house is sold and they move to London instead. He initially resents leaving the house he grew up in where most of his fondest memories were made, but ultimately the move makes it easier for him to forget Lupus.
1886 — Now nearing the final stages of his training, Theo is excited to become a fully fledged Auror and has gotten used to being his father’s heir. He has grown closer to Amity since Lupus’ vanishment but still misses his brother, not that he’ll admit it. As far as he’s concerned, Lupus is a selfish git who ditched his family. He’d be The Most upset if evidence of Lupus being dead turned up though.

He's recently made contact with his brother for the first time since his "death".