You don't exactly decide. At least in my opinion. Anyways, most of the time your favourite characters can grow on you, others you find you start to like over time, or in some cases a character you grow to like even though you didn't like them at first. Some times characters can become your favourite right away. -smiles-
The way Prussia became my most favourite character was an over time thing. At first I actually didn't like Prussia. I thought he seemed loud and kind of obnoxious. After awhile, I started to see his character more and his personality. He then became my most favourite Hetalia character. I absolutely just love Prussia and I admire him. He's utter perfection in my eyes.
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