In episode 10 of Kabaneri, Biba (aka. the Liberator) visits the local lord. He takes his Kabaneri with him, including Mumei.
As soon as they arrive, Mumei claims she needs to go to the toilet, then uses that excuse to run all the way back to the station's gates and open them.
What I didn't understand was why Mumei needed to accompany Biba in the first place. Since opening the gates was part of a grander plan devised by Biba himself, wouldn't it have been simpler to just leave Mumei behind?
Even from a purely political point of view, there doesn't seem to be any reason for Mumei to act. Unlike Biba, she's not a particularly important (political) figure; she wasn't born in a particularly rich or notable family, and although she calls Biba "brother", the two of them aren't officially related. Her absence would likely have gone unnoticed by the lord and his retainers. And even if her absence had drawn attention...
all of the witnesses would've been either murdered or kidnapped by Biba.
What was the point of taking Mumei along, then have her make up an excuse to run all the way back to the gates, instead of leaving her behind?