Taken from the SnK Character Guidebook 2017, page 37
Disclaimer: My Japanese isn’t great, so errors may be present in the translation. I am open to correction.
“I wanted him to look gentle and mild mannered, but I also gave him a button nose so he would have a defining feature. Characters in manga often have props to distinguish them, like a unique hat, because there are so many characters, but it’s difficult to notice his nose as a feature. I also made Armin physically smaller than Eren and Mikasa.
The name “Armin Arlert” sounds like “aluminium”, so it’s alliterative and easy to remember*.
In the TV anime, Armin’s parents made a balloon. I didn’t have time to show it in the manga, but I did draw a rough version of his parents. Clearly, Armin is similar to his father.”**
Similarities to Armin:
Mother- Hair, nose, eyebrows
*TN: aluminium in Japanese is アルミ (arumi) which is just one character off from アルミン (Armin).