Eren:

You met Eren when he first joined the Trainees Squad. You noticed he was struggling when it came to the test where you had to keep your balance. You offered to help him, and he was thankful ever since that day. He repaid you later by saving your life when you were about to be devoured by a Titan.

Armin:

You were friends with Armin ever since you two were younger. You had the same interests as him. He was there for you through all your tough times, and you were there for him. You would also always help him whenever he was being bullied.

Levi:

You had been trying to make it up out of the miserable Underground District. However, you could never afford the prices to get up there, until you met a man named Levi. He saw how badly you desired to reach the surface, and allowed you to join his gang.

Jean:

You were a good friend of Marco's, and Marco had decided to introduce you to Jean. At first, you didn't like him, but then the two of you got along pretty well. You also got into teasing him by calling him Horse-face.

Marco:

You met Marco when he was asking Eren about his experiences with the downfall of the Shiganshina District. You were just as excited as he was, asking questions left and right. The two of you had blurted out the same question in unison, making both of your cheeks go red. He later introduced himself to you and he apologized for blurting out the same question as you.

Sorry if these are super cheesy, I had Writer's Block for whatever reason while writing these. Anyways, if you want me to include any more characters, don't be shy to ask!