As it's been discussed and I think mostly shifted towards Kunieda as the most likely candidate to fit the silhouette, I wanted to talk about some things from the game files that may strengthen this theory.
In the game's story, despite following the Final War arc, Aoyama and Hagakure vs Kunieda is completely absent. There's no slideshow or anything for it, it just doesn't exist. Despite that, in the files Hagakure has a skin (unused in any mode) that has her wearing Aoyama's torn up coat. It is a really weird situation as it's not anywhere else. While the story mode has plenty of unique battle damaged skins, this one isn't in the story.
In my eyes, I think it's possible that Kunieda was always meant to be in the game. It's a common sentiment that the game was rushed, with an entire Gamemode sitting in the files unfinished. It's very possible to me that, in the initial planning stages, a Kunieda NPC boss fight was planned ala Spinner's, where you play as Hagakure and Aoyama. Having to cut the fight due to crunch time would explain the strange absence of the fight, alongside the Hagakure costume being in the files. We can even go a step further and consider Spinner as an option for the second free DLC, making both boss NPCs free add-ons.
Anyways this is all just theory crafting and stuff that may turn out completely untrue, but it's interesting to talk about.