This is just me posting what my 'state' was when i unlocked / released Attack Mode Imp. She had 70 takedowns, was at level 10, and i had completed all brothel talks regarding Imp. I had just finished the boss at the Library/mage stage and the "imp-2" talk had become unlocked at the brothel at THIS point as it was not available prior to the fight. After doing the imp2 talk event i was given an Attack mode imp at level 10 (the same as my defence mode imp, which was replaced by the attack mode one,; you lose your defence mode imp)
Now the variable im unclear on is the number of takedowns required to activate the talks. Its totally possible that i overshot the required amount needed to unlock but defeating the Mage Boss definitely seems like the "store line event" needed to unlock her.
Hopefully this will help someone confirm the requirements of unlocking.