Re suggestions for practical hybrid. Remember she is 12 years old and wants a mountain bike. Adult logic says a hybrid is more suitable but she may not be happy with a sensible "granny choice"!
Example - when she was 14 my eldest granddaughter was given a 20 year old Univega mountain bike, in excellent condition. Initially she refused to ride it, not because it was 20 years old, not because it was a rigid, not because it was a hand me down from her granny, but because it was fitted with slicks. All her Army Cadet friends, and school friends had MTBs with knobbly tyres - 14 year old logic told her it was not a proper MTB with slicks, even though she only ever rode on tarmac - once it was fitted with knobblies she loved it!