How to Unlock the Secret Ending
Collect All 100 Cat Shines before the Final Battle
You can unlock a secret “true” ending after defeating True Bowser at the Wasteland Giga Bell region. This requires you to collect all 100 Cat Shines throughout Lake Lapcat.
Defeat True Bowser (Final Phase)
Defeat True Bowser by riding Plessie on the water. Attack him using the three Giga Bells while avoiding his attacks. You need to hit him three times to defeat him.
After defeating True Bowser, the secret ending will appear after the credits, as well as a tally of how many Cat Shines you have collected during the entire playthrough (which should be 100 to see the secret ending).
Note that obtaining less than 100 Cat Shines before the final battle against Fury Bowser will only get you the normal ending afterwards.