I'm betting that the girl Mokou is talking to is Reisen or Keine. Another bet I'm taking is that the girl is possessed and will now take over Mokou's body to cause havoc in Eientei

I'm betting on it being Marisa again or Tewi. Mokou would have said "oh its you Keine" instead of just you, and she generally speaks in a different tone to Keine than someone like Marisa, but it may just be done to hide the fact it was Keine. Reisen is watching the Eientei so I doubt she would have gone over to Mokou in this time. Since we haven't seen Tewi at all and last we saw Marisa flying away on her broom to somewhere I would put my bets on them. Plus if it is Marisa again it would make even more sense that the spirit is Mima.