In my story the MC's dad died of disease 8 years prior to the story, and his death have something to do the Macguffin and how it cause his death, by that I mean he offered to die in the place of his son(MC's brother), but when of I write about him he comes off as "Too pure for the world, Manic Pixie Dream Boy" Marty Stu, I get that he is dead but is there any other way to flesh him out like giving him flaws or some other identity than MC-chan's dead dad, Like I'm not good with writing parents that much
How to make this guy more of a character and less of a glorified plot device
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What kind of meaning and involvement did the dad have in the MC's life? Perhaps, if the dad was absent a lot, the MC might look back on their dad's death as kind of meaningless, or maybe they were really close and the MC finds themself missing the dad a lot.
Well he was the more surporttive parent in the MC's life, and was drove to depression when he died
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So you could probably add in what the dad liked to do/activities that he did with the MC?