Alternatively it could be the character which has already been hinted at several times and explicitely shown a few chapters ago instead of characters that have done literally nothing indicating they might be guilty except by existing.
Absolutely hilarious how whenever there's a new character people mindlessly jump at saying they're the mastermind based on nothing but when it's the character who's been foreshadowed multiple times and told directly in this chapter, suddenly it's a red herring lol
Ueshiba Riichi is a hack.
It would've been unironically far more interesting to continue with the mom route instead of random romance #154286 but for that to happen, you'd need courage.
I mean, it could still be possible, riko said they'll considerate "Hello Abyss".
And hopefully riko decides to go with Made in Abyss indeed since it'd fit perfectly.