Suika is the magical girl who sealed Ancestor
In ancient times, Suika, probably one of the first magical girls, was the one who sealed Ancestor Lilith. Luckily for Ancestor, this was long before Suika decided on her 'just kill them all' mentality. Suika's core is inside Momo
Somehow, Suika can give Yuko a Poke in the Third Eye from within a Pensieve Flashback in Momo's mind. The most obvious solution is that Suika is trapped in there with them, inside the magical girl. That would also explain Momo's propensity for falling to the darkness, especially when Suika threatens to "return soon" and murder her.