Gawain was the middle of three sons born to the Llewellyn family, an upper-middle class family of Welsh purebloods - although by the standards of those who liked to keep track of things they were barely worthy of the title. Still it was enough to keep the family 'safe' during Voldemort's first rise to power and the attacks on Muggle and Muggleborns during Gawain's childhood. (This quiet acceptance and utilization of privilege by the family is something Gawain is ashamed of to this day. He insists that being a child does not excuse him.)
Right between his brothers, Gawain soon became the peacemaker, the healer of the three, and the one who dreamed big but quietly. He did his chores, kept things neat, and rescued injured animals by taking them to his mother, Rhiannon, who worked as a healer at St Mungo's.
As was expected, at eleven Gawain went off to Hogwarts. There the Sorting Hat sat on his head for a good five minutes as it deliberated between his cleverness for Ravenclaw and his willingness to work hard to make something of that cleverness for Hufflepuff. The verdict: Hufflepuff. There by the kitchens he spent his seven years, learning magic and making friends. Although he was not head boy, he was a prefect which suited him just fine. The same went for not being on the Quidditch team - with his terrible fear of heights, the stands were more than enough to scare him.
He graduated with excellent marks (nothing less than exceeds expectations) and was taken on to apprentice as a healer at St Mungo's. The apprenticeship would have been hard enough on its own, but a year later he undertook a challenge that had his magical friends really worry about him: studying Muggle medicine at Cardiff University. With magically forged education documents, of course. Muggle student by day, healing apprentice on the night shift was hard. During those years he had little sleep and no social life, but by the end of it was both fully-fledged healer and actual Muggle doctor.
Of course, that was the time of the second wizarding war, when Muggles and Muggleborns were under threat. As much as it hurt, he withdrew from Muggle medical practice to return to the wizarding fold as a healer who had finally awoken to the truth. He was in a better position to help the resistance alive and a liar, than dead and speaking the truth. Having now spent years living much as a Muggle enabled him to move seamlessly between the two, a valuable trait in any member of a resistance.
He survived, but not without cost. Friends were lost and he had seen horrible things, the dead and wounded on battlefields that he could not help. In one panicked moment he even went against the core of his being and took a life. It was that of a Death Eater who had turned on Gawain after he interrupted his torture of some Muggle teenagers. It was the Death Eater or Gawain and the teens, and logically it was the correct choice, but it was but one of many final straws.
After the Voldemort's defeat, Gawain packed up his things and moved almost as far as he could. There were several stops along the way, pauses to examine potential new homes, before he finally found a new place. Melbourne, Australia.
There, after using just enough magic to ensure a smooth acceptance of identity, Gawain set aside his wand.
He was not fool enough to snap it, nor to ignore the presence of wizarding Australia, but for all intents and purposes he lived like a Muggle. To them, he was a young, quiet Welsh doctor now practicing in a small clinic in the middle of a big city.
He didn't so much make friends as get adopted by the people next door who realised he only came and went to go to work or get groceries. One of them was a woman named Amanda, and after a couple of months of her flatmates inviting Gawain over (or inviting themselves over to Gawain's) she sat him down and asked him out. She was a Muggle, she sold sunglasses, and she was absolutely perfect.
When it came time, she proposed to him. Marriage, and exploring the world together soon followed - it was one adventure, not two, and soon parenthood followed. Only one soul Gawain has ever adored more than Amanda, and it came into this world angry and wrinkled and screaming. He was used to saving lives, and once he had taken one, but now he had helped make one. And with all the love he felt he would help shape her to be extraordinary.
They were in Germany when it happened. He was working in their magical hospital, she worked as a tour guide for English-speakers, and their daughter was not quite old enough to go to school with the great big cones that accompanied Muggle children to their first day of school. It was a mundane death - a car running a light, the deaths of both drivers instantaneous. One minute he was husband, happy; the next a widower with an enchanted ring that no longer beat in time with his wife's pulse.
They returned to Melbourne after that, to be close to Amanda's parents. For Gawain, the UK was not even an option. Instead they went to Japan, Gawain hoping to immerse Siwan into the culture of her heritage. They were there for five years (three years of her as a day student at Mahoutokoro, two years boarding; Gawain working as a healer assigned to the British Ministry Embassy) before the need to move once again came. Back to the UK this time, and Hogwarts for Siwan; with her boarding Gawain moved about, sometimes working as the doctor on an island in the Hebrides, other times living in central London to work again at St Mungo's. Sometimes he volunteered at Doctors Without Borders, returning in time to whisk Siwan off to Spain or somewhere for the summer while he did something else related to magimedicine.
But that was no life for a teenager, and the decision was made to come home to where it all began for her: her mother, her grandparents, her birth. She was sick of the moves, sick of the separation from her father even when they were in the same home. Although this will be her third school, Gawain is determined it will be the last - and as a teacher he can be available in a way that he was not in any other position. They are home... For good.