Armin Artlet has been the series' dandere archetype since the beginning. This personality type is introverted and soft-spoken, often with anxious tendencies and a kind heart. Of course, the shy blonde boy perfectly fits this description, but in an anime as brutal as Attack on Titan, a character like this finds survival far more difficult.
Growing up, Armin's innocent nature caused him to rely heavily on his friends, narrowly avoiding death on numerous occasions. However, since his debut in Season 1, the reclusive blond has clearly undergone significant character development. He boldly steps into Erwin Smith's shoes and employs his tactical mind to his advantage. Now, Armin is barely recognizable as the dandere best boy he once was.
Armin is not as physically powerful as his peers
Introduced as the crybaby of the series' main trio, it was clear from the start that Armin lacked any physical prowess. Mikasa is an Ackerman naturally gifted with the power of the titans, and Eren's resolve and dedication only made the third stick out like a sore thumb. Throughout his time in the 104th Training Corps, it became obvious that Armin would always be the deadweight among his peers. For example, when struggling on a weighted march, Reiner helped by offering to carry his pack, and he was the only central character who failed to rank in the top 10 during graduation.
Even as a full-fledged member of the military, others were still picking up Armin's slack. In the battle for Trost, when the group was fresh out of training, Eren had to sacrifice his own life to save Armin, and in return, the warrior shut down completely. Unlike other members of the 104th Survey Corps, he has never killed a titan and likely never will.
For a long time, Armin was thought to be useless because, even when he tried his hardest, he could barely hold his ground against the weakest opponents. When he was protecting an unconscious Jean Kirstein from a puny 3-meter titan, for example, all he could do was screech and swing his sword around ineffectually. The pair were not attacked, but Armin's attempt at protection was inadequate. However, this version of the warrior appears to have vanished entirely.
Armin compensates for his intelligence deficit
While Armin still lacks brawn, he more than makes up for this with his mind power. In his training, he often struggled with physical tasks but was most capable in the classroom, putting his brain to work. His innate aptitude for combat tactics played a part in Levi's decision to save Armin over Commander Erwin Smith in Season 3, as his fresh outlook and military genius made it clear he could easily match his predecessor's excellence. Armin has a cunning mind and as such is one of the series' most important characters. Without his leadership, the Survey Corps would be lost, as no other member possesses the same mental skills that come to Armin so easily.
Armin not only has a natural understanding of military and war strategy, but his manipulative tendencies allow him to sway situations in his favor. The best example of this was when he was confronted with the similarly manipulative Yelena. When she announced Zeke's euthanasia plan, he began crying, but these were crocodile tears used to make the opposition lower their guard and trust the prisoners. Armin isn't above lying to the enemy to further his cause, and it's a strength that always works in his favor.
Armin is no longer dependent on his friends
Until his character progression was revealed in Attack on Titan's fourth season, Armin had no shame in relying on Eren, Mikasa and other members of the 104th Corps to keep him out of danger. Now, he leads them as the 15th Commander of the Survey Corps and assigns them tasks and roles. Since the day his life was chosen over Erwin's, Armin has felt the pressure of his predecessor's legacy. He has stepped into his leadership role with bravado, redeeming the death of Erwin and impressively leading without the bias the previous commander had.
He lost the luxury of using Eren as a crutch when the protagonist began to spiral and thus had to branch out as an individual. He even stood his ground and confronted his best friend, punching him in the face when he insulted Mikasa when previously he would have stood by and blindly trusted Eren. Armin is no longer the shy boy who lacks strength and determination. Now, if anything, Eren and Mikasa look to Armin when it comes to big decisions and solutions to problematic situations.
Armin Is the Colossal Titan
Armin, as a human, obviously lacks physical prowess, but after inheriting the Colossal Titan from Bertholdt, his useless position as a soldier changed. Armin is clearly best used behind the scenes of battle, but as the titan, he is crucial to Paradis Military's offense. He destroyed Marley's port and ships for Eren, and he can easily take on any rival titans.
The most infamous of all is the Colossal Titan, who destroyed the walls, leaving a trail of death and destruction in its wake and setting off the events of the series. Armin, as the heir to this titan, possesses the island's most valuable and powerful resource, one with the potential to wreak more havoc than any member of the Survey Corps. He has a brilliant mind and a natural ability for leadership and command, and he is Paradis' most valuable asset.
Overall, Armin has matured into himself, gaining newfound confidence and undergoing a transformative character arc. He is the moral compass and fearless commander of the 104th Survey Corps, and he has developed into a leader who can more than compete with the former. Despite the fandom's displeasure with Armin's selection over Erwin, Armin has more than earned the title, demonstrating that he isn't the dandere archetype he began the series as.