Here's my OC for the series. 

Name: Kazumin Banjou 

Age: 2,000

Looks: At his normal height, Kazumin stands at 6'8, taller than most humans but still dwarved by giants, though this is a 'normal' height by shrinking himself down, and his full height is 15 feet. He has a slender build, but he does have some muscle definition. Kazumin has green eyes, one of which has a scar through it, but thanks to some healing it's mostly superficial with no lasting damage, and his eyes are always shining with life and passion. His hair is brown, and always messy. 

He dresses very casually, just with black armor for his legs and lower body. For his upper body, he wears a button up shirt, but with the buttons all undone, and he wears an undone tie, just for the aesthetic of it 

Sin/Holy Knight/Goddess/Etc: Kazumin is half-giant and half-human. He considers himself to be a Sin and has given himself the marking of one, however, this is unofficial, and he can best be considered a good ally to the Sins, and someone who does travel with them, similar to the role Elizabeth takes on

Powers: Due to his mixed heritage, Kazumin ended up with an amalgam of his mother's giant abilities, and the magic his father had as a Holy Knight. From his mother's side of the family, he has basic earth-based abilities, similar to Diane on a lower level, and weaker versions of magic like Heavy Metal, and the strength, stamina and endurance given to him by being a giant. From his father's side of the family, he inherited light-based magic, able to project beams of light and projectiles to attack, or when he concentrates to make a hard-light construct, like a wall to protect him and others. 

Kazumin also has the ability to alter his size on his own, though it's not nearly as effective as when someone like Merlin uses a spell, he can go from his normal height to his full height at 15 feet, or shrink himself down more. 

Sacred Treasure/Weapon: Kazumin asked for a Sacred Treasure, but he was never given one due to not being an official Sin. Instead, he goes through a lot of weapons, mainly gauntlets when he fights, and he's still asking for a Sacred Treasure. 

Pairing: Diane 

Backstory: Kazumin's mother was a powerful giant warrior who had an affair with a Holy Knight who they had captured, which lead to the birth of Kazumin. While his mother had considered this to be a loving relationship, the father left as soon as he could since he already had a family to get back too. This led to his mother being angry and bitter, and Kazumin was someone who she could take it out on, abusing him when she could. 

Along with this, Kazumin was an outsider among the giants, due to his mixed blood. He was the runt of the litter, which led to him being picked on and taken advantage of due to his comparatively small size, and the fact that due to being a human he was aging faster than the other giants, and so at the first chance he got, he left the Giants and began to strike out on his own in the human world 

In the human world, however, he had the opposite problem, he was bigger than most people, he was stronger than almost anyone he came across, and while the people he would come across aged, he would not due to the slower aging of the giants. On top of this, he also was able to have a human sense of time, so he was aware of how much time passes for others, and that he didn't belong in that world either. 

Kazumin began to feel like he was in a loop, as the people around him would grow up and start to get married or start their families and become adults he was still more or less a teenager. The repetition of his life began to drive him crazy, and so he had to do something to break up the month, and he also had years of pent up resentment from his time being abused by the Giants. 

This led to him traveling the world as a traveling fighter, garnering a reputation for himself, as someone who would appear in town, beat their strongest member, and continue on his way. However, Kazumin was unable to truly be happy this way, or find a place where he belonged. This changed when he met Meliodas and the Sins, and Meliodas beat him in a fight. 

Meliodas didn't think much of it, but for Kazumin this meant a lot to him. It gave him a goal to work too, getting stronger than Meliodas, and also a place he felt he fit in, with the other rambunctious and loud Sins. From then on, he would continue to follow them, challenge Meliodas to rematches, and spend the time with the Sins. 

When Meliodas and Elizabeth would begin to put the Sins together to save the Kingdom, Meliodas did decide to bring Kazumin along, figuring he would be happy to help 

( Let me know what you think, and then maybe in the comments you can leave me your OC, or tag me to something, and then we can find a place to start)