Prediction: we’ll eventually see Cosimov as an Akuma

Remember Cosimov, the Toxic clown? The guy that made Allen's life hell back at the circus? I find it interesting that he was given a special ammount of focus for a side character, which is rare in this series, having been an important source of trauma for Allen and for that matter Mana/the Earl and having alluded to a backstory by mentioning his noble blood.

If he really is a disowned nobleman he likely does have a family that could mourn his death if they found out of his passing by say...a former co worker of his who now happens to have become an "Earl" and therefore connected to high class circles. Thus he makes the deal to return him to them in order to make him an Akuma in order to push human Allen's buttons and perhaps even getting some payback for killing dog Allen by torturing his soul.

It would also be quite fitting for him to become the base soul for the first level 5, becoming a personal nemesis to Allen. Then again, he could be part of Allen's ongoing flashback, perhaps being the first Akuma he fought and the one his memories were rewritten to believe was Mana and the one who started the trend of Allen pittying Akuma.