Armin from Attack on Titan rarely turns into the Colossal Titan. In Hajime Isayama’s stories, the Colossal Titan is important because it was the first “shifter” to show up in Attack on Titan, and it’s important to him.
This is how the story starts: Bertholdt Hoover, one of Marley’s crooked soldiers, is in charge of the Colossal Titan at the start of the story. Attack on Titan season 3 is the last time Bertholdt has that kind of power. Armin Arlert outsmarts and defeats him. Armin is saved thanks to the help of Eren and Mikasa. Armin eats Bertholdt and gets the Colossal Titan for himself.
Since then, we know that Armin used the Colossal Titan to protect Paradis Island from Marley’s ships during the pre-season 4 times skip in Attack on Titan. Armin also used the Colossal Titan’s destructive power to destroy a Marley port during the attack on Liberio in the fourth season of Attack on Titan.
He blew up the surrounding area, then trampled everything that was left under his huge feet. Weirdly, Armin hasn’t changed at all since then. Most Titan-shifters change their form for each battle (Eren’s Attack Titan, Reiner’s Armored Titan, Annie’s Female Titan, and so on). Armin, on the other hand, usually prefers to wear his 3-D maneuver gear and join the soldiers instead.
At times, Armin has forgotten about his Colossal Titan powers. In Attack on Titan season 4, for example, Armin forgot about his Colossal Titan powers when he fought the Marley invasion and the Jaegerists at the port (for which Reiner and Annie both transform straight away). One possible reason is that having a Titan that is so big and cumbersome isn’t very practical.
Because the Colossal Titan is so big, you could argue that it isn’t the best weapon for most situations. Armin can’t risk destroying the Azumabito clan’s “flying boat,” or hurting nearby allies with the Colossal Titan’s burning steam in the port battle.
Armin Transforms into Colossal Titan
Even if Armin didn’t Titanize during the Shiganshina fight, which took place on a much bigger field, that doesn’t explain why. Also, Bertholdt used partial transformations to get around Colossal’s size problem (the head and shoulders atop Wall Rose in Attack on Titan season 2, for example).
Another reason is Armin’s unmatched brainpower. For some reason, Levi didn’t want to give life to Armin over Erwin. He thought Armin was a Titan-killing machine, just like Mikasa. Armin is valued for his intelligence and strategic skills more than his strength, even if his strength is as powerful as a nuclear bomb.
Being the Colossal Titan would make it much more difficult for Armin to communicate and read the battlefield. It’s not like Armin couldn’t do both at the same time, though. We saw this in Attack on Titan season 4.
The practicality of becoming a 50-meter, burning hot monster, and Armin’s value as a military strategist may partly explain why he doesn’t want to use the Colossal Titan. But that’s not the whole story. Since Armin took the Colossal from Berholdt, Arlert, a young boy, has felt a lot of guilt.
Many times in Attack on Titan season 4, Armin says that Erwin should have been saved instead, and he wonders if he actually deserved the second chance his Colossal Titan gave him. Through the Paths, Armin has also started to feel for Bertholdt, which makes killing Marleyans morally hard. The fact that Armin feels bad about destroying Liberio’s port is well-known. Armin would rather not feel that way again.
It’s probably because Armin is a tactician and his Titan form is so big that he doesn’t have as many transformation scenes as other shifters. In Attack on Titan, Armin doesn’t use his monster form very often because he’s ashamed of how he became the Colossal Titan and how many people have died because of it.