Chiyo's senpai is a friend of Chiyo who, like her, wears the team's hat and comes to cheer at games.
Putting on their hats, Chiyo and her senpai watch the game together.
In episode 13 she calls Chiyo on her cell phone and they sit together. From their conversation, it is revealed that Chiyo used to play softball.
She then chides Chiyo about whether or not she has a "favorite" player amongst the male team. Chiyo shyly avoids this inquiry.
Central doesn't appear to have her voice actress credited in the end of episode 13 so figuring out her name is difficult.