As the owner of a 6* Mr. Negative...
Is there something I'm missing? He seems fatally underwhelming. His basic attacks are, naturally, really weak. The dev spotlight made him seem like he had incredible degeneration potential, but that's not exactly true.
Let's break it down for a moment.
The most critical thing I think that's dragging Neggy down is that the Light and Dark energy start expiring too soon. The devs talked about how the ideal rotation is cashing in with a few furies and a 20 dark energy L2, but that just isn't possible. Unless you're on a node like Icarus that will constantly be giving you buffs, Light Energy expires way too quickly to build it to 20 to convert. The best I've managed is 12-13 in testing so far.
On top of that, let's go through a simple calc.
Assuming no Fury buffs (and his furies are honestly rather poor as well), a single degeneration deals 2586 damage at 1/25. Let's add another 12 on there, assuming I got up there through perfect play. Our maximum damage output for an L2 is whatever damage off the actual attack is plus...33618 damage.
That's *okay*, but plenty of champions have access to that same kind of damage without having to jump through hoops in order to get there.
I will say that he has an absurdly easy access to perfect parry, which is a neat and easily spammable trick at the expense of some damage.
As a war attacker, I think he leaves a fair amount to be desired, as plenty of characters do plenty more damage doing far less, and his kit requires somewhat higher skill with micromanaging light and dark energy timings. As a defender...well, Nick Fury kinda steamrolls him, so there's that little issue.
So then, what's my consensus?
Cool as hell character, some viable utility, damage was way overblown in the champion spotlight.
Suggestion: Just push back the expiry start time to 15 or 20 charges. It wouldn't make him broken and would make him much easier to use.