Per Wiki Policy, moments pages are Spoilers Off. You have been warned.
The fight (if you can even call it that) against the Student Pilot is heartwrenching, especially for anyone well-versed in mecha media. He's a bright-eyed, hopeful rookie who dreams of becoming one of the big shots and is called upon to pilot a Super Prototype in a crisis - pretty much ticking off all the boxes for a protagonist in nearly any other mecha-based anime or game you can think of. But he's in ArmoredCore, so he never even comes close to achieving that dream, there's nobody to save him from being unceremoniously shot down by 621, and it's not even for any sort of personal reason - 621 just happened to take up a mission to destroy the mech he was piloting. All those hopes and dreams, snuffed out for the sake of a payout.
It isn't hard to feel sorry for the Rubicon Liberation Force. They're no more than colonists who want Rubicon 3 and the Coral to be left alone, understanding the dangers it poses after the Fires of Ibis. The Corporations, driven by greed and the prospect of fortune despite being aware of the dangers, send their personal soldiers and hire mercenaries to attack the RLF to get at a valuable resource. Worse, it's made abundantly clear that these people are not soldiers. They're normal civilians who resort to using whatever pieces of scrap lying around they can get their hands on to protect their planet and its resources.
Pretty much everything to do with the Liberator of Rubicon ending. You have to watch as characters you've probably become attached to and have helped you throughout your journey die, mostly at your hand, for the sake of the greater good.
First, you kill "Cinder" Carla and "Chatty" Stick, the latter of whom has been nothing but polite and courteous to you. If you kill Carla first, Stick, a rather stoic AI, shows anger in his voice for the first time and declares you're in a world of trouble for killing Carla. If you kill Chatty first, Carla slips into a cold rage of her own, trying to hold it together as she continues fighting you on her own.
"Cinder" Carla: Times like these...I can't help but laugh!
Later, during the second to last mission, V.IV Rusty shows up in a shiny new mech called Steel Haze Ortus, with a new emblem without the muzzle on the dog. He's severed ties with Arquebus and decides to help you. And then right after the boss of the level, he is unceremoniously Killed Offscreen.
And finally, the last mission. A one-on-one duel with you, vs a Coral powered AC, piloted by Handler Walter. He's undergoing Sanity Slippage due to being utterly inundated with Coral, implied to be a result of Arquebus capturing him and subjecting him to their "reeducation" program based on dialogue from V.II Snail in both fights with him leading up to this fight. Walter spends the battle talking to himself about how he has to finish his friends' mission of destroying all the Coral, and deliriously praising 621, saying they've "earned all the credits", and how they can "finish the job, reverse the surgery" to live a better life, all while Ayre is on the side begging for the two to stop fighting. After a tense duel, he refuses to back down, charging up his cannon while 621 stares him down, muttering about how he HAS to do this. And then, he stops. He slowly lowers his weapon, and says in a genuinely warm tone...
Handler Walter: Look at you…621…You’ve found…a friend…
Rusty's last words in the Fires of Raven penultimate mission, Breach the Karman Line, seem intentionally designed to cut as deep as they possibly can:
The "Fires of Raven" ending. One of the bleakest and bitter endings in an Armored Core game, it kicks off with 621 deciding to go along with Walter's plan and create a second Fires of Ibis — the very same calamity that ravaged a planet and other nearby planets while robbing billions of lives in the process. Walter is willing to go to extreme lengths to make it clear to the known world that Coral, as a resource, is better off left alone.
While the obvious loss of life alone is horrific enough, it gets worse when you remember Coral is not just a resource, but a collective mass of living organisms. Among those organisms is Ayre, who has been nothing but kind and thoughtful of 621 throughout the entire game. She's understandably upset you'd want to enact mass genocide, but it's far worse given that Ayre genuinely considers 621 a friend, if not more. The final battle with Ayre has her coldly and sadly lament how her bond with 621 is now gone.
Ayre:...I realize what you are. The spark of war...the fires that haunt Rubicon! (...) Humanity wants to burn it all! Everything on Rubicon! I won't let you...Your fire must die!
That's not even going into how your friendship with Rusty deteriorates to the point he enacts one last desperate charge against you in an attempt to stop you from carrying out Walter's plan.
It hurts even more that, as Ayre expires, her last words are her still clinging to the hope of the shared dream humanity and Coral being one due to her prior friendship with 621 — wanting desperately to believe that you'll still do the right thing and you won't destroy the Coral in her last moments. As you complete your genocide both of the Coral and the entire Rubicon system, you are left to wonder if it really was worth sacrificing your Only Friend who was willing to stick through it with you thick and thin, as well as millions of lives, to serve your debt to help Walter and earn your freedom.
