As the series goes on, Luffy seems to (in certain fights) obtain some kind of (SUPER AMAZINGLY SUGEEEEH~!! but most times temporary) battle form
Examples:
Mizu Luffy
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Afro Luffy
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Gear Second Luffy
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Gear Thrid Luffy
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Nightmare Luffy
!
Candle Champion Luffy
!
My personal faves are Afro and Candle Champion