Any character that can fight is a warrior. Their battle can either be a literal one or a metaphorical one. A literal fight involves physical combat and is the type of fight I usually think of warriors going through when I think of this type of character. A metaphorical fight on the other hand involves fighting against non physical things such as greed, envy, or society itself. The battle can even be between the character and that same character’s mind.
Because of the many forms of battle a character can go through any character can be a warrior. Of course there will still be differences in skill level. The difference between a person who has only just picked up a gun for the first time and an expert sharpshooter is a great one after all. Keep this in mind when you write such a character.