Theory time (spoilers ahead for Once In A Lifetime and Eternum)
(Sorry it's a long read)
What if the true ending of once in a lifetime is the one where you go back through the sewers instead of confronting astaroth because it shows you in a cell and you get visited by Astaroth who talks about wanting your help to find calypso and if so you may be able to get your mind back, we know that the two games are connected not just by this but also a few references like president stabb (stabby mike) being mentioned near the beginning.
That's the solid theory so far but that's not too concrete and doesn't really mean anything other than Astaroth wants to find calypso. However what if The Founder is Astaroth because yes Astaroth has different hair colour and style but it's not difficult to imagine that he can alter his appearance and acts differently, this could also explain why he has the ability to grant the item that allows players to keep items no matter what.
Some might think that last one is a stretch but I believe that it could be true based on what we know so far about Eternum however this is all just speculation and could be proved completely false by the next update(s) of eternum or by Caribdis themselves excited to know you guy's thoughts on my theory.
TLDR: Astaroth wants calypso and might be the founder based on evidence so far and we may meet the player from Once In A Lifetime based on what we know.