@Veridis Quo: There is only one Fixer (that's why it is capitalized: it is a name/title given to that individual). He is the Grand Master's* tool and the person the other people in Zerul interacted with and Ixion briefly even fought; the general public and even most officials have no idea who he is or, for the matter, that he even exists.

*The Grand Master is a demon lord.

Posting order:
Jack in Jillian's storyline and borderhouse collab. (And probably the encounter with Kaedan, too, though I'm still willing to take it upon myself as I originally offered, should you find yourself handling too many lines at once.)
Legion in the other collab. 
Mercinus in Zerul. 

Unrelated:
Are avatars broken for everyone, or just me?