My three main Hawkes would react pretty differently to the death of Fenris.
Lyla (diplomatic warrior) is one of the few "goody-goody" characters I've created that I actually like. She's very lawful, very supportive of the templars because they are the law, and while she is not hateful or cruel to mages, she is positive that the Circle is where they belong. With that said, she approached Fenris with compassion and patience, and their relationship leans toward the softer, more trusting side. (She's the one "Serving" was based off of, where Fenris gives her a massage.) As the Viscount, she depends on him to be her oasis of normal/good/calm after she spends hours arguing with nobles. If she lost him, she would slowly lose her ability to focus until she eventually gave up her position, left Kirkwall and wandered for the rest of her life, trying to do good. Under the right circumstances, she could eventually recover, but she wouldn't be the same.
Vivian (aggressive blood mage) would pretty much lose what little bit of control she has over her temper and dive right off the deep end. She'd probably raise her own army and wipe out a few countries until her death finally caught up with her. Recovering would not be something she'd ever want, so it wouldn't happen.
Reese (sarcastic rogue) is a bit...touched in the head to begin with. She drinks, she frequents the Rose (when Fenris isn't around), she does incredibly stupid things in combat that really should have killed her a hundred times over. She can't take anything seriously, is terrified of her feelings for Fenris even though she can't live without him and won't take sides on anything. The only reason she supported the templars in the end was to spite Anders. (Her reaction to being named Champion, and later Viscount, was something like "lolwut?") If she lost Fenris, she would drink herself to death, or failing that, she would find a way to commit suicide-by-guardsman. She would never recover.
Holy wall o' text, Batman! D: Happy 1500 pages! XD