Hattori is a ninja working under Ieyasu Tokugawa. She is first seen in Season 1, Episode 7.

Spoiler

In episode 7, Hattori attempts to ruin Hideyoshi's play as ordered by Ieyasu by secretly changing out Nobunaga's script. However, after thinking that it wasn't enough, she then decides to act on her own accord and set the curtain on fire. Unfortunately, the curtain catches on fire quicker than expected and begins to burn down the entire stage. For ruining the whole thing, Hattori is promptly punished by Iyeasu.

Source: the official site (http://sengokuotome.com/character/index.html)