A Symbiote in Shingeki No Kyojin

It was the day the titans reached humanity's line of defense— the walls. Titans poured in and devoured many while some survived. But not all were lucky to survive and escape this dilemma. A child that got covered in ruins is slowly dying. Nothing can save his life now. Except, of course, if something like a symbiote from another world comes in and helps him...