Secret Bambino is summoned by Positive King on floors 4 through 7 of Yukiko's Castle within the Midnight Channel. They will be summoned as long as the Positive King is not accompanied by a Phantom Mage or another Positive King. Selling three of the Poison Flowers they drop at Daidara Metalworks will unlock the Poison Kunai weapon for Yosuke.
Persona Q2: New Cinema Labyrinth
Secret Bambino appears in Area 3 and Area 4 of the second Theater, Junessic Land. When defeated, it may drop Bambino Leaf and Bambino Crystal. Bambino Leaf is used to create the Sequoia Mace and Femur Club for Ryuji Sakamoto, the Shooting Star for Morgana, the Long Stick for the P3P heroine, the Titanium Club for the P4 hero, the Great Bird Fan and Hanachirusato for Yukiko Amagi, the Drunkard Claw and Tyranno Claw for Teddie, and the Camo Poncho unisex armor. Bambino Crystal is used to create the Stellar Saber for Goro Akechi, and the E-Collar armor for Morgana.