A Sister's All You Need

After Haruto tells him how he feels about Miyako, Itsuki promises to help him out. Around the same time, Nayuta, who sees Miyako as an older sister, tells him that she wants to have a birthday party for her. Itsuki and Nayuta have difficulties trying to figure out what to get Miyako for her birthday, so Nayuta asks her who her favorite author is, and she responds with Itsuki. Nayuta feels jealousy toward Itsuki for the first time, since they’re both authors. What was the gift that Nayuta ended up getting Miyako?