When the main characters of The 100 first landed on Earth's post-apocalyptic ground, many of them were nothing more than lost teenagers. Because of the strict laws on the arc, many of them were criminals, having spent the majority of their lives in a jail cell called the Skybox.
Though some of the characters arrive on the ground with some skills (see Raven Reyes and her engineering skills, or Clarke and her medical experience), for the most part, these characters start their stories with no particular expertise... but by season six, they've shaped up to be the perfect Dungeons and Dragons adventuring party. Let's see what classes these characters specced into.
Madi Griffin: Battle Oracle
Madi was surviving the Earth's second apocalypse when she met Clarke Griffin, who became a mother figure for her. As the last Nightblood, it was Madi's destiny to join with The Flame and assume the throne, ascending to Commander herself, and that's exactly what happened in season five.
Madi's martial skills increase once she has access to the Commanders' wisdom inside the Flame. Madi didn't choose the Flame; it chose her, and with Gaia's help Madi was able to grow into her role as Heda... that is until Sheidheda took advantage of Madi's innocent nature, forcing Raven to destroy the Flame and revert Madi into a plain low-level fighter.
Gabriel Santiago: Loremaster
Though the other Primes seem to have, well, let themselves go career-wise, Josephine and Gabriel were unique in that they never stopped learning. Though Josephine mainly kept her studies focused on the inner workings of Sanctum, Gabriel did a ton of research while in exile. As a result, he became a sort of Loremaster on Alpha, keeping secrets about the functionalities of the forest around him even from his children.
When Octavia's arm is caught in the anomaly, he becomes obsessed with the potential research to be done, forsaking his vow of "Death to Primes" so he could help Octavia remember what happened inside the anomaly, which he calls Alpha's greatest mystery.
Emori: Rogue
Initially a thief, Emori proves throughout the show that she's so much more than John Murphy's girlfriend. When she gets stuck in space with the rest of Spacekru, she takes advantage of her time there by learning engineering from Raven. She even learns how to operate and fly a spaceship, and in season five she successfully lands a dropship on Earth by herself.
A tinkerer at heart, Emori can remove Murphy's shock collar after he's caught by Diyoza. When the red sun makes her homicidal, and when she attacks Murphy she displays a martial prowess that indicates she also sparred with Echo in space.
Echo: Master Spy
If Echo had a dollar for every time another character called her a spy, she'd have a bunch of paper that was essentially worthless to her—but people do call her a spy a lot. After Azgeda raided her village when she was a young girl, she was found amongst the ash of her burned home and mother. The Azgeda Queen raised Echo to be an assassin and spy. Echo served the Azgeda Queen until her son Roan assumed the throne, and then she served him too.
Through sheer luck, she's able to group up with the rest of Spacekru and escape to the Arc after Roan is killed the second apocalypse begins. Though there was no need for a spy in space, Echo immediately resumes her role in season five on the ground, when Octavia sends her to spy on Charmaine Diyoza.
John Murphy: Rogue
Also known as the cockroach, Murphy is a master of survival. Though when he lands on the ground he has no skills and a horrible attitude, Murphy quickly adapts after he's exiled by the other members of the 100. Where Clarke excels in diplomacy checks, Murphy excels in bluff checks.
And though his main skill may be deception, he's also handy with using firearms, brawling, and sensing motives. He's even a good cook! In season six we see his experience in deception pay off as he manipulates Russel and Josephine into making Emori and him immortal before betraying them to save his friends.
Gaia: Monk
Almost a cleric, Gaia is the last Flamekeeper, a religious leader similar to a priest in the religion of the Grounders. As a monk, Gaia excels with a sword, staff, or spear, and serves Blodreina as the main combat instructor for her group of novitiates. She learned how to fight from her mother Indra (the same person who taught Octavia) but instead of becoming a warrior, she devoted herself to Becca Pramheda and the Flame, and through her belief, she finds strength and power.
In season six, after she is exiled by Madi and Sheidheda, she decides to return, choosing to serve her faith at large instead of to obey her Commander, which a cleric would likely never do.
Raven Reyes: Technomancer
In season six, Raven rephrases a famous quote and tells her fellow adventuring party members that highly advanced science is indistinguishable from magic—and that is a very good way to summarize Raven Reyes' entire plotline. After being controlled by the AI A.L.I.E. Raven finds herself with the ability to understand a coding language made by Becca Pramheda.
Not only can Raven read the language, but she can also code in it. Though she claims all of her prowess came from the "City of Light Community College," it's clear to see Raven has always been skilled with technology, and it's only been enhanced by the AI. Not quite a wizard, Raven Reyes is a technomancer.
Octavia Blake: Fighter
In season one, Octavia Blake is so adamant that she be trained in combat that Indra takes her on as a second and teaches her the art of combat by sword. She uses that sword to serve King Roan as Skairipa, a brutal assassin feared by the people of Polis. Clad in sleek armor, she wins the conclave with just her sword and her wits, which leads her to the bunker where she ruled Wonkru by that same sword.
The leader of the grounders has always been a warrior, and Octavia is no different. Though in season six we see her drop her sword for less brutal tactics, she shines in the season finale as she calls out battle orders to Bellamy and Echo while they keep the tavern from being set on fire.
Bellamy Blake: Brawler
Bellamy has been instrumental in the survival of the human race; he's a natural leader and fighter, and whatever side he's on is normally successful while he's on their side. In the early seasons, it seemed possible Bellamy had a rage ability with the way he handled opposition and conflict, but eventually, he mellows out.
After Bellamy and the rest of Spacekru are stuck on the Arc for six years, Bellamy returns to the ground an expert in Azgeda fighting styles. Though Bellamy was trained on the Arc in the usage of firearms, and in season five we see he's grown proficient enough with a sword to spar with Octavia, Bellamy prefers to fight with no armor and his bare hands, which makes him the brawler of this D&D adventuring party.
Clarke Griffin: Rogue
Clarke lands on the ground with a large amount of medical knowledge, and Earth skills she learned from her father and Pike, such as using a map. She's very focused on survival from episode one when she admits she won't stop to admire the beauty of being on the ground (her people have never breathed fresh air, for example) because she's worried about radiation and lack of supplies. Presumedly off-screen, Clarke learns some combat skills.
At the beginning of season three, we see her slay a panther with nothing but her wits and a knife. In season six, she single-handedly takes out all but one of the primes. With such a large, non-combat heavy skillset including diplomacy and chess-level strategy—not to mention her sharpshooting skills, or her ability to find and set traps, Clarke is undoubtedly a rogue.