Reiner treated Armin like a princess. 

After training he would go up behind Armin and pick him up bridal style and swing him around.

Eren thought nothing of it and laughed along with everybody else when Reiner started to sing horribly.

Armin would get all flustered and start waving his arms frantically and start squirming to try and get out of his grasp.

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Whenever Armin would be stressed or have trouble with something Reiner would swoop in and yell "Do not be distressed, your knight in shining armor is here to save you!".

One time Ymir accidentally bumped into Armin causing him to fall and Reiner caught him like this

They were super close to each other's faces and everyone would just stand there awkwardly watching their blank faces. 

Reiner smirked and leaned in to kiss him but Armin started panicking and punched him in the face.

The punch was so hard that it knocked Reiner out.

Armin kept apologizing to him the whole day.

Reiner said the only way he was going to forgive him was if Armin gave him a kiss. 

Armin was a blushing mess.

Armin quickly kissed Reiner's forehead and rushed back to his own bed to go to sleep.

Reiner has never let him down and constantly brings up the kiss causing Armin to start babbling nonsense.