Very late plotting post, ahaha.

For Levi (cleanlevis) his most strongest sad memory would be from his past when he lost Farlan and Isabel in a battle against titans - seeing them die right before him. And then seeing his whole squad dead after Female Titan battle one. What do you mean positive memories?

For Ichiru his memory is most likely one of the times when his parents kept talking behind his back that he's too weak to follow Zero when he begins training seriously to become a hunter. The other option would be Shizuka-sama disappearing in his arms.

For Loki (divineliar) his memories are pretty much from his past (when he was a small kid) when everyone ignored and hated him. That or when another god wanted to kill him and Balder saved him - which is a bit bittersweet of a memory.

I also have Takuto (itsudemoissho), Hori (kashimapls) and Yosuke (captressentiment) if someone wants to plot with them for anything o/