It depends on which 'book' you mean—if we're going by the traditional 24-book division of 'The Iliad', Book 16 is the Patrocleia, where Patroclus begs Achilles to let him fight. The key roles there are obviously Patroclus and Achilles, with their whole doomed friendship dynamic driving the action. Patroclus puts on Achilles' armor and turns the tide for the Greeks, but he gets carried away, disobeys the order to just push back the Trojans, and goes for the walls of Troy. That's where Apollo intervenes, Euphorbus gets a spear in, and Hector finishes him off. So Hector's a major player too, since he kills Patroclus and takes the armor, which sets up Achilles' return later.
But honestly, I think people sometimes overlook how important the gods are in this book. Apollo is the one who knocks Patroclus' helmet off and weakens him, and Zeus is watching the whole thing, knowing it's all part of his plan for Achilles to re-enter the war. Even Sarpedon, a Lycian ally of Troy and son of Zeus, gets killed by Patroclus earlier in the book, which prompts Zeus to consider saving him but Hera talks him out of it. That divine layer adds so much tension because you see the characters as pawns in a bigger game. So yeah, the key roles are mortal—Patroclus, Achilles, Hector—but the divine actors like Apollo and Zeus are just as crucial to how the plot unfolds.
What always gets me is the shift in tone. Up to this point, Achilles has been sulking in his tent, but Book 16 is where the consequences of his anger really spill over onto someone he loves. It's not just a battle scene; it's this heartbreaking moment of misplaced heroism. Patroclus thinks he's helping, but he's actually stepping into a fate meant for someone else, and you can feel the tragedy building with every line. That emotional weight is why these characters stick with you long after you finish reading.