I suddenly realized how downright perfect it was to have Saint Patrick as Oisin's Natural Enemy. Since I depicted Patrick as a guy obsessed with creating a 'utopia' through Christendom and Oisin as a guy who's seen a so-called 'utopia' and was all "Eh, it's not all that", it totally works to have them as rivals. The philosophical fight-banter practically writes itself!
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Added more detail into Oisin's personality section reflecting what I just wrote up above. Really nice when you work out a really good character conflict without even intending to.