Ok, so we know that Mr Fixer used to go by Black Fist.
Looking at the comic numbers, he started in "Justice Comics" after the first appearance of the Bunker suit (possibly GI Bunker), but before the "Mystery comics" run (where the Freedom 5 and their members are featured).
This led me to believe that he was around in the 70's or thereabouts. Now, correct me if I'm wrong- but a kung-fu hero in the 1970's called "Black Fist" sounds pretty appropriate. It was the era of classic martial arts movies and blaxsploitation films!
Doing some research about comics of the era, I started thinking about Iron Fist and Power Man. They got their start in the 70's, individually and as the "Heroes for Hire" team.
So... What if Fixer had a partner, once upon a time?
Thinking about it, that partner would likely have skills that complimented Fixer. If we are keeping to the 2-man team trope, the partner always has a different skill set, but with some overlap and never one that completely overshadows the other. You don't see a duo that comprises a person who can re-arrange matter on an elemental level and one that is really good with knives. theres always a measure of equality and a reason that they are partners.
Fixer is all about technique and control (and he's blind). Theres a LOT of potential options for a partner there.
Black Fist and (something)! Cleaning up the streets!
So what would happen to the partner when Fixer decided to walk away? It seems conceivable that the partner would be left in the lurch. Maybe he'd keep fighting. Maybe he'd quit. Maybe he'd get in a fight and badly beaten.
Maybe he's been living with that bitterness for decades.
Then he sees Fixer alive and well, and seemingly in the prime of his life. Or maybe he gets an offer in the mail with photos of Fixer alive and well, and a promise that he too can have his youth and vigor back.
We know of at least two people with access to a chemical treatment that rejuvenates and enhances. Both of whom have a grudge against Fixer. One of whom was alive at the time Black Fist was in operation, and might well remember a partner...
Picture it- Fixer hears the voice of an old friend, someone that he hasn't seen (figuratively speaking) for years. Maybe theres some guilt there at leaving his friend.
They talk for a while. Fixer goes in for the arm grab, or the manly hug...
and then stabbing!
This could explain the nasty little smile on heartbreaker's face. The way he got in close to one of the best fighters alive without any injury. the way Fixer looks so surprised.
And somewhere the chairman is laughing.