Depends on the context, or whose thoughts Kobayashi is focusing on primarily during the chapter. Most of the characters are somewhat flexible and have been both a protagonist and an antagonist at some point. For example, Eri and Yakumo have both antagonized one another before (though, Eri spent the majority of the time in the role as protagonist).
Whoever fights monsters should see to it that in the process he does not become a monster. And if you gaze long enough into an abyss, the abyss will gaze back into you. - Friedrich Nietzsche