I think The Reapers goes with both Mystery and Presence. They still make me wonder where they came from, what they originally were programmed for etc. Each time a Reaper shows it's ugly head, I nearly s**t my pants. Imagine the mess at the end of ME2. Still, in ME2 we learned that they live only to create more of themselves. Motive.

"There may be growing flowers on my legs, but you've forgotten to tie your shoelaces!"