Negaduck is Darkwing's evil twin from another dimension known as the Negaverse. He is considered to be one of Darkwing's most dangerous arch-enemies and one of the few characters with zero redeeming qualities.
It should be noted this is the second version of Negaduck. The first Negaduck appeared exclusively in the episode “Negaduck” as a tron split version of Darkwing Duck.
Not much is known about Negaduck's origin, aside from the fact his “secret hideout” is another dimension known as The Negaverse which is assumed to be his birthplace.
Negaduck is a Grade-A Psychopath who enjoys being evil for the fun of it. He is happy when everyone else is miserable, or dead (or both). There is no point in time where Negaduck is revealed as having an emotional weakness, nor does he seem to care for anyone.
That said, he does have a more humorous, sarcastic side and will occasionally pitch immature temper tantrums, similar to Darkwing. He is known to throw out a pun or two while causing immense pain to others and genuinely has fun when he's wreaking havoc.
Negs also fancies himself as somewhat of a Casanova. In the episode “My Valentine Ghoul” he thought he could use his charm and good looks to win over Morgana Macawber in order to use her magic for his diamond smuggling plot.
Negaduck loathes Darkwing with every ounce of his being and will not rest until he has destroyed every aspect of his double's life. Negaduck is arguably one of Darkwing's greatest foes, and has proven himself to be a capable threat. That said, Darkwing often does not take Negaduck seriously which leads to a lot of back-and-forth sassing.
He is the leader of the Fearsome Five and despite being physically weaker than the rest of its members, he maintains authority by instilling fear in them. His repeated abuse has earned him no loyalty from the other members, and as a result the Four are not opposed to forming a truce with Darkwing Duck after one of Negaduck's betrayals (“Jail Bird”).
Little is known about Negaduck's relationship with NegaGosalyn, aside from the fact that he is her legal guardian. There are no direct interactions between the two in the one episode she appears in (“Life, The Negaverse, and Everything”). By the end of the episode, NegaGosalyn is adopted by the Negaverse versions of the Fearsome Five (known as the Friendly Four).