ok, i'm gonna say it: octavia learning to swordfight for like 2 weeks and then somehow becoming such a "master" and "warrior" is unrealistic.
your OC shouldn't be perfect at fighting by the end of season 1. of course, they'll probably have some experience with guns if they're a gunner, but their overall skills may be sloppy and based on luck/adrenaline.
keep in mind that grounders have been training to fight for their entire lives. it was revealed in 5x09 that lexa won the conclave at the age of twelve. most of the warriors we see are fully grown adults and the delinquents are under the age of eighteen. your OC shouldn't be winning every single battle they face without some form of help or rescue.
i'm trying so hard not to constantly use O as an example because this isn't supposed to be a rant about her BUT... just don't use her as a model because, in retrospect, it isn't very believable.
it's much more interesting to read about a character who learns from their mistakes, grows, and changes into a better fighter than one who just magically is from the start.
not everyone in the show wins their battles. for example: (possible spoilers ahead, so they're marked by season)
season 1: bellamy only beats dax with clarke's help
season 1: bellamy is saved by finn
season 2: octavia loses a fight with a grounder
season 2: the 48 lose a fight with the mountain guards (jasper shoots and misses)
season 3: bellamy gets knocked out by roan
season 3: team kane stage a coup and ultimately lose
season 3: octavia and monty only beat hannah by killing her
season 4: octavia loses fight to echo
season 5: octavia loses fight to bellamy
those were just some of the examples i could think of. anyways, remember that your character is supposed to be human, not an all-powerful superhero who never makes mistakes!