She notices that something is off when she overhears Kichou talking about the “old” Nobunaga when Micchi and Saburou switch places during the New Year chapter/episode. Later, during a festival Micchi gets a coughing fit and Kichou sends Yuki after him to make sure he’s OK, so she learns that Micchi and Saburou look the same (she doesn’t tell anyone, though). She starts being suspicious of the whole situation, but doesn’t put the pieces together until much later, when she eavesdrops on a conversation between Saburou and Micchi which makes everything clear. So she’s pretty much the only person, aside of Saburou, Micchi and Takugen, who knows that Micchi is the real Nobunaga.

Since at this time she’s under orders to assassinate Nobunaga, she tries to kill Saburou… but of course she can’t do it because she loves him. She ends up leaving Kenshin’s service, and enters the Oda household for real this time, and Saburou asks her to protect Kichou.

So far her storyline goes kind of parallel to everything, and aside of accidentally destroying the history book she hasn’t really contributed much to the overall plot… so I think she’s just there because a) the author likes her, and b) in the end she’ll be the one to protect Kichou (someone needs to, after all).