There’s a total of seven characters in the game, you just have to finish a game playing as them. You do not have to win. This achievement will probably come naturally when going for some of the other achievements, but if you want it quicker, then host a ranked hunt match. When it starts, pick your character and play the first round. Once the round finishes, end the game. Once you get back to the main menu then you should find that it counted towards the achievement.
It is recommend doing each character in order, because the game has a feature that tells you how many characters you have finished a game as, but it doesn’t tell you which characters. If you don’t play them in order then it will be much harder to keep track of.