You and Armin had been friends since childhood along with Mikasa and Eren. But like some groups, they fell apart. So you and the others never really talked to each other until you guys all joined the Survey Corps. Armin still didn't like you but he still knew he was your friend at some point.

Today was his 21st birthday and Everyone knew it was his birthday but decided to celebrate without him. Even Annie, his girlfriend. He was so upset that you found him crying against a tree