It looks like Nightwing has a secret weakness after all, and it was exposed by Harley Quinn of all people. During an escapade in Gotham, Harley found the one thing that could throw the usually calm and collected Dick Grayson off of his game.
Once Robin the Boy Wonder, Dick Grayson grew up cutting his teeth in the life of superheroism by the side of Batman. While Batman had a reputation for being the grim and dour Dark Knight, Grayson essentially served as his opposite. Dressed in brightly colored clothes and always bearing a smile, the young Robin was Batman's needed counterbalance. Even when Dick outgrew the sidekick role, he kept that freewheeling attitude and charm he carried with him as Robin. While few members of the Bat-Family can be as serious as Bruce, Nightwing stands out as one of the most peppy allies of Batman.
But just because Dick is usually all smiles doesn't mean he isn't aware when a line has been crossed, and one boundary may be his biggest weakness yet. In Harley Quinn #11 by Karl Kesel and Terry Dodson, the hero swings by his girlfriend Barbara Gordon’s apartment only to discover Tim Drake and Cassandra Cain are having difficultly wrangling in a criminal. Nightwing immediately responds to his allies and discovers what's causing the fuss: Harley Quinn flitting around Gotham dressed as the Barbara Gordon version of Batgirl. Seeing Harley disgrace Barbara's outfit turns Dick apoplectic, and he goes berserk trying to apprehend the not-Batgirl. However, in Dick's rage, he allows Harley the opportunity to steal his motorcycle, forcing Nightwing to commandeer a private citizen's car, not realizing his new ride can't handle the same turns as his own bike.
It’s a rare sight to see Dick lose his cool like that. Nightwing is seen as Batman’s more amiable and less prone to anger counterpart. But clearly seeing someone make a mockery of Barbara Gordon's role as Batgirl is something he can't abide by. Harley's brief costume change was most definitely a sore spot for Nightwing, as this issue came out when Barbara stepped down from the Batgirl role after being shot by the Joker. It may have been unintentional, but Harley accidentally discovered Nightwing's most critical weakness.
It’s probably not the most shocking reveal that Nightwing cares deeply for his allies and loved ones. Dick’s dedication to the people he cares about is what helps Nightwing hold the Bat-Family together. However, as Harley Quinn found out, that dedication can be used against him. By stirring up emotions he has over Barbara’s shooting, she was able to get a major edge on him. Nightwing wouldn't ordinarily leave his motorcycle running or steal a car to chase after a criminal, but Harley's stunt had Dick seeing red. If she was actually aware of the personal relationship between Barbara and Dick, Harley Quinn might have found the perfect ammo to always defeat Nightwing.