Completely unsupported prediction that will likely be wrong:

For a sidequest, we will meet a Sin Eater who has somehow managed to retain their consciousness and sanity, and who needs quite a lot of help. Everyone else avoids them due to their appearance, and they do have immense power that they are trying very hard not to abuse, but they are still quite cheerful upon meeting us and knowing that we are willing to help them.

Like all Sin Eaters, this NPC will have a grotesque appearance. Their most prominent feature would be their head, which is basically a sphere with two small round eyes and a creepy grin, and they will lead us to the 24-man raid series.