Sorry for such a late reply, I keep forgetting and then doing other things, oops. Well, I was thinking maybe Eren or Levi (doesn't really matter to be honest) could slowly be going blind at the age of... 15? The other becomes really good friends with them, and tries to help them cope: making sure he remembers how different things look. This carries on for 3 years until the one going blind breaks down, saying things like "I can't do this" and "I just want to see clearly again" as the other tries to comfort them. Carrying on, the other soon tells him "you might lose your vision, and if you don't remember how anything else looks, just remember me." After a while, he eventually does lose all of his vision, but the other is there to help him out... Just a little thought, not sure if it makes sense.
