Mr. Whammy is a 77 year old season ticket holder and Nets eccentric enthusiast whose “hexing” is a staple of home games (as the article states). His wife has a matching “Mrs. Whammy” Jersey that makes them the most adorable Nets couple (sorry Jay-A and Beyonce). You can find Mr. Whammy cameos on “The Association” Episode 5 in the clips from the Knicks game.
To add to what William said, Mr. Whammy has followed the team for a very long time (I believe his real name is Bruce Resnick. His seats are located behind one of the baskets and during opponents free throws he likes to hex them, as William said. During the first game at Barclays center though, security asked him to backup and that he wasn’t allowed to stand in his usual spot. I guess that initially he was upset, but soon after the issue was presented to “Nets people” (Irina Pavolva), he was able to stand in the spot and was left alone. They also gave him his very own Brooklyn Jersey that says “Mr. Whammy.”