I think the one confessing to Keine is "futatsui"wara no "mami"ko.
Sumireko has the Occult ability of a Doppelganger. That's what Mokou is talking about and it's why Mokou was asking Sumireko to do something rather than go through the trouble of burning it herself (because if it were just Mamizou, she WOULD have done something). Notice that she remarks on Keine's ability to see past it like she knew this meeting would happen all along and she was simply conducting a social experiment.
If you play other characters against themselves in ULiL, some will remark on the existence of a doppelganger urban legend in the victory quotes. If you play a Sumireko vs. Sumireko match, however, she declares that it is her ability.
It's impossible to read into if you only look at her character's wiki page. You need to dig into the matchup scripts themselves.
I knew Keine would notice.Keine, I love you.I feel like you're acting differently to how you usually are...Nope. Why don't you go out and beat it yourself?By the way, you erase that damnable doppelganger right now?Was I that far off?Um...
