I'm not sure about Armin being the most layered character... Could I have your reasoning? (He is a very layered character, just not the most in my opinion)
Everything Armin has been through just makes him very human, in my mind. I literally don’t understand how people don’t like Armin, and I’m being dead serious here. I mean, with some characters I understand why they get criticism. Mikasa is overprotective, Eren is a bullhead, Levi and Jean both have tendencies to be pricks, etc. But Armin? I literally don’t see a fault with him.
Everything he’s been through has impacted and added to his character in some way, and made him the most complex and realistic character in SNK by far. At the beginning of the series, Armin was weak of both mind (I mean confidence-wise) and body, held back by his own refusal to see himself as useful. Even to the point where he basically told others to let him die instead of him because that’s how little value he saw in his own live VS everyone else’s.
After her put his true talents, his god-tier mind, to work and realized he had support from his friends, Armin’s character underwent a drastic change. He gained confidence in himself. After seeing so many people he knew and loved dying senselessly to the Titans, he came to his conclusion “to make change, you have throw aside your humanity.” After that, he became downright manipulative, in a very ruthless and cruel way. And he’s done it more than once.
When he was nearly raped while impersonating Historia, Armin’s darker side truly started to emerge. He realizes now that the world is horrible, and to make the world better, he himself had become a horrible person, alngo with Eren, Mikasa and all his other old friends.
And finally, despite all this, he was still sicken to the core when Armin had to kill someone to save Jean’s life. Despite all he’s been through, and how dark he’s become, Armin still retains some of his boyish charm and personality that made me fall in love with him from the very start. He is an amazingly deep and fleshed-out character and is a testament as to just how good at characterization Isayama really is.