Talk to the fork standing outside the shop in Inkwell Isle 3 to get a hint about getting A's in the game. He will now appear in the final phase of the fight as a blue angel, helping her with attacks of his own. The stage will collapse and crush Sally's paramour in the background. Then, stand on the opposite cherub angel, and both platforms will collapse. Stand on the cherub until you hear a cracking sound effect.
During the first round, parry off the heart-kisses to jump onto the cherub angel platforms on the left/right corners of the arena. Secret Boss 3 - Sally's Sweetheart (Sally Stageplay): Go to Isle 3 and fight Sally Stageplay. Djimmi will reveal a Puppet Cuphead that fights alongside him in the end of the fight. Once you reach the third round, transform your airplane into the smallest form. Secret Boss 2 - Puppet Cuphead (Djimmi The Great): Go to Isle 2 and fight Djimmi The Great. The Radish will appear with the Carrot for the final round. After a certain amount of time has passed, a Radish will instead appear. During the battle, defeat the Potato but do not attack the Onion that appears after him. Secret Boss 1 - Little Radish (The Root Pack): Go to Isle 1 and fight The Root Pack. After downloading the 1.2 Update, complete the indicated task to fight the corresponding secret boss: