Hen is the most competent person on the show, which means she gets the least to do in Season 1. Her arc—struggling with her medical exams while her wife Karen wants a baby—is the "B-plot" of the B-plots. But watch her eyes during the rescue scenes. She is the only one who sees the trauma clearly. She is the heart of the 118, even if the script hasn’t given her a crisis yet.
Before the intelligence, before the trauma, Buck was simply chaos . Season 1 Buck is insufferable, horny, and reckless—and that’s the point. He steals a firetruck for a date. He tries to sleep with Abby while actively flirting with her rival. He is a liability. The brilliance of the writing is that we see his vulnerability only in flashes (his estrangement from his parents, his desperate need for Bobby’s approval). This pack is the "before" picture of a man who will later be broken and rebuilt.
Angela Bassett does not do "supporting character." Season 1 gives Athena the most grounded, rage-filled arc: discovering her husband Michael is not only having an affair, but is in love with a man. The show doesn't shy away from her homophobia or her violent fury (the scene where she destroys the closet with a baseball bat is terrifying). She is a cop who uses her badge to intimidate her husband’s lover. She is morally gray, and that’s what makes her great. The "copaganda" aspect is present, but balanced by her personal implosion.