There are many different characters and backgrounds in The 100. The main ones are the Hundred and the Grounders, of course. Most OC's have the same background that way, having been simply from Trikru or the Ark, which is understandable because they have the most information and face time on the show, but I'm here to explain that there are other backgrounds they could have.
i. the boat people
The boat people are mentioned in season three. Luna, a nightblood who fled from her conclave, lives on an oil plant in the middle of the ocean with many other people. They're ones of peace and not war, unlike the other clans.
You could bring a character in in season three, when Clarke, Bellamy, Octavia and Jasper go to the oil rig in search of Luna. When the people are slaughtered by ALIE, the character could become enraged and join Clarke the others to stop ALIE.
Or they could simply die. Your decision.
ii. mount weather
Of course, most people don't want to make their OC from Mount Weather because they die fairly quickly, but even so you could make a story with at least sixteen chapters with an OC like this.
Hell, you could make a fanfiction for Maya with a sky person.
iii. ice nation
Azgeda is ruthless and make an appearance in season 3, so you could make the OC come in then. They could be Echo's second, someone loyal to Roan even after he was banished. They could have been banished together.
iv. miner
Like Shaw and Diyoza, other Miners could be on our side. The OC could have had a similar past as Charmaine or something— maybe they're Shaw's sibling or best friend? Maybe they're a defector from Wonkru? You never know.
I hope this helped give you new ideas!
