"This is our exclusive Mona Tart!" (当店限定モナタルトです) by Mahiru Yurono
Link: https://www.pixiv.net/en/artworks/99944878
Link: https://www.pixiv.net/en/artworks/99944878
Link: https://www.pixiv.net/en/artworks/108833831
Not to get spoiler territory. The first time you go into battle, Yukari drops her gun and the main character picks it up and without any knowledge or prompting takes the gun and shoots themselves in the head, which summons their Persona. It was shocking and brought back a bunch of memories from playing this game as a teenager.