It was not commonly known that Armin knew all the lyrics to pop songs on the radio. Most would not suspect him of even listening to them to begin with, given his demeanor towards just about anything even dealing with pop-culture. Sticking to his books in the quiet of the isolated part of the park is what Armin is known for.
Sometimes Eren or Mikasa would say something and a single word or a phrase would set the other off in song. Armin would correct their lyrics, but never join in himself.
When asked by his closest friends about this out of place knowledge, he simply groaned loudly and stated it was the work radio, the one in the bookstore he worked at, that was at fault. He just knew the lyrics after a few listens, three at most if the song lyrics were hard to distinguish.
Eren Jaeger was the only one who knew that Armin would sing them when he thought he was alone. This is knowledge he treasured deeply.
The first time it happened Eren had just gotten home to his apartment, which he shared with his best friend, and he heard it before he opened the door.
"His favourite colour's green. He likes to argue, born on the 17th. His sister's beautiful…"
Armin's voice stunned Eren as he stood outside his door, leaning his head against the door. Not even thinking to be quiet, Eren unlocked to door and opened it.
"If you asked me if I love him, I'd-"Armin cut off as he heard the creak loudly.
Eren's immediate response to the silence was asking him why he stopped.
Armin blushed and walked over to the couch, where he curled up with a book, silently fuming to himself.
Now of days, Eren has learnt that stealth is the true key to listen to his best friend sing. Eren loved how Armin sang, and then of course he locked himself in the bathroom for a bit, looks in the mirror and tells himself that his friend singing does not turn him on.