Fiona's demeanor is grumpy and aloof, and is also pretty sensitive so her moods tend to spike and drop. She's fine up until something she's insecure about (her height, general size, feelings for Bailey) comes up, which is when she becomes defensive. Fiona can also be pretty rude and blunt, and in a rage she can be childish. Her speech is rough and informal whether or not she's trying to make herself seem bigger. She likes to fight and is surprisingly good at it, though she probably should stop.
But in truth, Fiona just cares way too much about her friends and is able to put her own fragile feelings aside to put her loved ones first when it really matters.
She loves quiet mornings, being left alone, climbing, reading, and video games. What a shocker.