What counts as a F/O is different for everybody.

At it’s very most basic, an F/O can be any character that you have a crush on, or feels important or close to you.

Many people call a character that’s extra special and important to them a ‘comfort character’. But if you feel like you love them, or have a crush on them, or you want to ship them with your OC or anything else, you can consider them an F/O.

With f/os there’s not any hard and fast rules.