First, give them a solid backstory. For example, if your OC is a sniper, they could have grown up in a rural area where hunting was a way of life. This gives them their shooting skills from an early age. Second, think about their personality traits. Are they quiet and reserved, or more outgoing? Maybe they're a bit of a joker to relieve the tension during missions.
You also need to consider their motivation. Why are they part of the Ghost Recon team? Is it for revenge against the cartels because they lost someone? Or is it a sense of duty to protect the innocent? For physical appearance, you can base it on real - life military personnel or use your imagination. Just make sure it fits the rough - and - tumble world of Ghost Recon Wildlands. And don't forget to give them some flaws. Nobody's perfect, and a character with flaws is more believable.
Another important aspect is their relationships with the other characters. Do they get along well with everyone? Or are there some team members they clash with? Maybe they have a mentor - like relationship with an older Ghost. These relationships can add depth to your character. Also, think about how they adapt to different situations in the game world. Are they quick to improvise or do they rely on strict plans? All these elements combined will create a more believable OC for your fanfiction.