I'm hoping her introduction is bringing us closer to Mamu-chan finding out Fujiyama is Sexy-chan.
I think she already knows. In Ch 13 wyen told that there's a suspicious person lurking about the school, she thinks about Sexy-chan raiding the classroom and then getting arrested, then thinks about Neighbour, and then leaves to intercept Sexy before she can get arrested.
It wouldn't had occurred to me she was trying to protect Fujiyama in that chapter, but I can see why you would think so.
Still, I hope Mamu doesn't know since I kinda like the "liar revealed" trope.