Zsh -oextendedglob, 82 69 51 bytes

<<<${1//(#m)(_?|[A-Z])/${${MATCH[2]:u}:-_$MATCH:l}}

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Even if the flags count toward byte count, adding 14 bytes for -oextendedglob still undercuts the 82 byte program!

<<<${1//(#m)(_?|[A-Z])/${${MATCH[2]:u}:-_$MATCH:l}}
   ${1//              /                           }  # replacement
        (#m)                                         # enable $MATCH parameter
            (_?|[A_Z])                               # "?" matches any character
                         ${MATCH[2]:u}               # substitute 2nd char, uppercased
                       ${             :-_$MATCH:l}   # if empty, substitute "_" and lowercased match
<<<                                                  # print to stdout