From Kannagi, she’s Jin’s childhood friend and guardian. While Nagi surely charms most people who watch Kannagi, including myself, not to be overlooked is Tsugumi.
First, she’s cute, which never hurts. Then of course there’s her personality. She’s known and taken care of Jin since they were both very young, but now that they are older she has begun to have feelings towards him, and she wants him to notice her as more than just a childhood friend. This makes itself readily apparent when Nagi suddenly shows up and is living with Jin. Tsugumi, almost to her own horror, declares to herself that Nagi is a rival, and she secretly rationalizes to herself all the ways in which she is better than Nagi, even though they are friends.
And there’s Tsugumi in a nutshell. She’s cute and just a little bit shy, and that’s why I like her.