All characters besides Buttercup, Cass, Daemon, Zerocool and Nidhoggr need to be buffed
Buttercup: is a mess due to having crowd control and insane damage this would be ok if she wasn't as insanely fast and tanky so I recommend either nerfing her mobility or her damage output.
"Tanky" is Buttercup's thing though, so I suppose she'd either have to be a Damage character which would require buffing her B ability or her mobility would have to take a huge hit which goes against her character. A compromise I'd be willing to see is that she can have either the mobility or healing from Burl, but not both unless it uses up two seperate mod slots.
I think the problem with Buttercup is that she's multiple character archetypes rolled into one. Control is fine, mobility is fine, tanking is fine, self-healing is (mostly) fine.... as one character? Not so much. Maybe when more characters that fulfil her roles are introduced nerfs can be more realistic.
Cass: her ability to deal insane amount of damage with her ult blade dance she can destroy all non tanks health and with healers who are underpowered she can't be stopped.
I've not seen enough enemy Cass players who use Blade Dance to comment on that, but the part about being with healers is obvious to me: you need to take out the healer. The same with most characters or competent healers. Cass is squishy on her own and her mobility encourages hit and run tactics. I've found when the team covers her exits she dies faster than any other character using that same tactic.
Daemon: is balanced besides my one problem with him which is he has to much health combined with his mobility which allows him to escape almost any situation.
I don't play Daemon so I'm not going to argue for or against this change. Anything I can say applies to all characters, which begins with "In the hands of a good player...."
Zerocool: he outclasses all other healers and his healing beam is debatebly more powerful than Nikos life force ultimate which makes his healing the only good healing in the game.
If it's true that Miko's Ult is weaker than ZC's heal then that absolutely needs to change, but I'd say buffing the Ult is more important than nerfing Zero Cool's heal. I suppose the problem then comes from how much waste in the form of overhealing comes from the Life Force ability, unless it slowly decays from the (now) extended max health of the receiver.
Don't forget that Zero Cool is a single target healer. Miko and Kulev have area-of-effect (AoE) healing in some fashion with cooldowns. Personally I like that ZC players are high value targets and when they die the party starts to sweat.
Nidhoggr: he deals way to much damage and when combined with Zerocool he's basically unkillable his ride the lightning ability affects way to much space around him and leads to him being able to stop a while team from moving instantly.
When combined with Zero Cool any character is virtually unstoppable unless all four are giving their undivided single-target-ability attention to said character. Personally I've never had problems with Nidhoggr unless I've been cornered by other, much faster basic attacking characters. I've found ranged characters are the best foil to his slow but heavy hitting melee attacks.
There's some good feedback there though, I'll look into the characters you list and see if I can agree or find other points of contention. ^^