Secret Bambinos are summoned by Positive King shadows of floors 3 and 4 of Yukiko's Castle, with up to three being able to be summoned at one time. Upon being defeated, they may drop Poison Flowers. Selling 4 of these to Daidara will unlock the Poison Kunai weapon for Yosuke Hanamura.
Secret Bambinos are summoned by Positive King shadows on floors four through seven 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 3 of the Poison Flowers they drop at Daidara Metalworks will unlock the Poison Kunai weapon for Yosuke Hanamura.
Secret Bambino appears in Area 3 and Area 4 of the second Theater, Junessic Land. The Bambino Leaf it drops is needed to unlock the Sequoia Mace and Femur Club for Ryuji Sakamoto, Shooting Star for Morgana, Long Stick for the P3P heroine, Titanium Club for the P4 hero, Great Bird Fan and Hanachirusato for Yukiko Amagi, Drunkard Claw and Tyranno Claw for Teddie and the Camo Poncho unisex armor. Its Bambino Crystal drop is used to unlock the Stellar Saber for Goro Akechi and the E-Collar armor for Morgana.