Momo had a slip of the tongue in this chapter. She's been aware of Keigo for quite awhile.
Literally admitted watching him as a kid. So I take it she fell for him at some point in his youth.
He eventually marries a woman that isn't Momo. Momo:
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Later on, that wife gets killed off. Momo:
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Now she just needs to drill the fact that she's legal into Keigo's thick skull.
But poor Momo... Edward Elric understands.