Kosuna, real name Koizumi Taiko, is accepted Kanta's apprentice only after showing Kanta a picture of a large breasted woman she claims to be her mother and promising him she will look like that in a couple years. She's young and flat-chested (Sunabozu has repeatedly teased her about it and she hates huge chested women because of it.). Seemingly unable to complete any task by herself, she's surprisingly an able assistant to Kanta.