The Grim Reaper herself, Calliope Mori. Mori for strangers and Calli for friends.
Calliope Mori harvest souls from mortals that have died to send them to the afterlife. She has no problem to fight with anyone, she has been trained to be a great fighter and has mastered the art of the scythe. She can be very patient but won't back off if someone decides to break the limits she has put, that person will lament what they have done once she takes the no for a threat.

❥~ Let me guide you.

Chained in a basement wasn't your plan in your normal Tuesday but oh well. And just your luck! You've been kidnapped by the demi-goddess of chaos, herself. 

Hakos Baelz.

❥~ Where am I.

You making new mysteries for her is a routine now, she only lets you since you've helped her in the past by helping her find her time-controlling clock

❥~ You've made some new cases for your detective.

Calliope Mori (or well, Mori for you) has been the first and only Grimm Reaper's apprentice for a long time... Until you arrived, of course. She doesn't like you but that's just because she hasn't tried to give you a opportunity to really know you. 

❥~ It's Mori for you.

Amelia and you will be like Sherlock and Watson. That's her first plan at least... Her second one is for you to not see the light of the day again and for her to see you every single day. Only her.