Animals can exhibit individually repeatable variation in behavior (i.e. personality), which may impact their fitness. A potential personality trait with clear implications for fitness is birds’ parental aggression against nest predators. Dependent avian offspring experience the highest predation risk during their course of life, and consistent patterns of parental aggression can be important mediators of the young’s fitness during these early stages. To test this hypothesis, we explored variation in parental aggression over the course of four years (2016-2020) in breeding pairs (n = 37) of Sali (Aplonis opaca), a locally endangered passerine on the Pacific island of Guam. Sali on Guam experience high rates of nest predation and fledgling mortality due to an invasive predator, the brown treesnake (Boiga irregularis). First, we quantified variation among breeding pairs of Sali in aggressive behavior towards human observers disturbing the nests, and found that an aggregate measure of aggression varied greatly among breeding pairs. Second, we calculated the repeatability of aggression as an index of personality and found that aggression was highly repeatable and consistent within breeding pairs, providing clear evidence of a pair-level personality trait. Finally, we explored the potential fitness consequences of variation in aggression for both nestlings and fledglings. We found that parental aggression was not correlated with either mean nest success or mean fledgling survival per pairs. Our findings suggest that although breeding pairs exhibit clear personalities with respect to aggression, this variation in personality is decoupled from offspring fitness, likely due to the lack of its effectiveness against the brown treesnake.