Node.js, 61 bytes

Contains the unprintable character \x7F.

s=>s.replace(/_.|[A-Z]/g,s=>Buffer(s[1]||''+s).map(x=>x^32))

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JavaScript (ES6), 72 bytes

s=>s.replace(/_.|[A-Z]/g,s=>s[1]?s[1].toUpperCase():'_'+s.toLowerCase())

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