Just my take on the Secret Boss for Chapter 3

This is Flowery! Flowery the Flowtertainer™!

Inside this huge mascot costume is Petals! She's a fun little flower who was the host of a now cancelled Kids Show because she kept trying to push her own visions of a violent action show into it, thus leading to "The Higher Ups" trapping her in the Flowery Mascot Costume!

Her Boss Fight in her final form would be similar to Undyne using the Green Soul Mode. She'd be tossing thorns and other attacks your way.

My idea for the upgraded Green Soul Mode is that you'd be able to counter green attacks! You'd have to time it with [Z] but you can also just block them normally. If you counter a green attack successfully, you'll get a small amount of spare %.

Petals also has an attacked called "The Super Awesome Can't Miss Beam™"! It's basically a one-hit kill no matter what, but you can enter this fight with or without the Shadow Mantle. Getting hit by it or not having the Mantle is basically an instant Game Over. But if you have the Mantle, you can counter the beam. It'll have more room to be able to counter compared to other green attacks, but as long as you have the Mantle, you'll be left with at least 1 HP if you get hit still.

After defeating Petals through sparing her, you'll get the Flower Crown.

After Defeating Petals through fighting her, you'll get the Re-Run Blade.