The primary characteristic of a secondary consumer in an ecosystem is that it primarily feeds on primary consumers, which are herbivores that eat producers (plants). Secondary consumers can be carnivores or omnivores, playing a crucial role in maintaining ecological balance by regulating herbivore populations. They contribute to energy transfer through the food web, impacting other trophic levels and overall ecosystem dynamics. Understanding secondary consumers helps ecologists assess food web stability and the interplay between different species within their habitats.