I've been seeing some complaints about chapter 5's secret boss, and want to address them with my reading of it all. Spoiler warning for Deltarune Chapter 5.
The most important thing in my view is to put Mad Mew Mew into context of the secret bosses as a whole. That being the fact that the Secret Bosses represent an arc for our characters, which I will roughly model using the model of the five stages of grief. Call this Grief Theory if you want I guess.
Jevil is in denial. He claims he's free, despite being in prison. He can do anything - but he can't. He can use all the bullet types, but his attacks are always in the same order, strictly confined by the code. All of his chaos is actually ordered. Even the piruettes are in a strict sequence, when it would have been easy to instead impliment some sort of RNG mechanic instead. The point is that all of his chaos, all of his freedom is false. He's just putting on a circus play.
Spamton is filled with anger at every injustice he's suffered through. He yells, he shouts, he punches. When his initial plan of posessing the NEO body fails, he lashes out at Kris, trying to take the SOUL in hopes it would offer him freedom (something we know isnt the case, since the SOUL is a shackle around Kris' neck.)
Chapter three is a little bit more complex, given that I consider its "secret boss" to the Mantle minigame, represented by the character of Ramb. Ramb is bargening. He is desperatly trying to make Kris, the person who holds ultimate power over his fate happy, to the point of self-destruction. It lets him hold on a bit longer, lasting until after the Shadow Mantle fight instead of turning to stone sometime between board 2 and your return to him. One could consider this a kind of victory, perhaps. Alternatively, one could also consider the fight against the Knight to be a kind of bargening. An attempt to defy the inevitable fate laid out by the Prophecy, one that isnt exactly a failure, but isnt a success either. Ralsei had hope the fun gang might have done it, that they stopped fate in its tracks, but they failed. One can also see chapter 3 in general as being about bargening, what with Tenna seeking to defy his fate of obselesance via making a bagin with the Knight via Kris...a plan one can argue as having been a success on the "pacifist" route, with Tenna's life being saved, in a semi-defiance of the prophecy.
Chapter four features Gerson being the goat and defying the model...or at least, brushing outside of the sides of it. One can argue Gerson is either denying depression...or, that he represents catharsis, which is included in some variations of the standard KΓΌbler-Ross model of grief. The moment that defies your path towards lasting depression by instead pushing you towards acceptance. Catharsis represents a regaining of hope. Without Gerson's advice, the gang has no reason to believe Susie might be able to change fate, putting them on the road to depression.
Chapter five then, is about acceptance. And with this model, you can see why Pink is the perfect character here. Pink is a character whose mind and body are struggling against each other, with the body wanting something the mind thinks is foolish, while the body is being restricted from exercising their emotions freely by the mind...but unlike the other secret bosses, Pink has a happy ending. Even Gerson doesn't get to continue living. Pink get to move to our Castle Town. And Kris gets told that there is a path forwards towards cooperating with us, if only they can talk to us the way the Body got to to Mew Mew, with the both of us accepting a compromise to get what we both want.
A small side note about the Shadow Crystals being Broken Dreams. In this reading, the Knight may not have even had one on them in chapter 3, with it instead being generated by our failed attempt to stop it. Pink has one because Deltarune's Light World lacks magic, so Mad Mew Mew is unable to fully become one with her body in the Light World, this only being possible in the Dark world due to it having magic. So Mad Mew Mew cannot live as herself in the way she wished to originally. However, she accepted this and instead chose to live in the Dark World, where she can be happy as part of Pink. This notably also eliminates the need for the Knight to have given any of the crystal holders except Gerson their crystals. Perhaps Gerson's is from Alvin? Gerson defies the notion his dream was broken, however. Which is why he is unaffected.
Now...there is one more elephant in the room with regards to Pink. That being that Mad Mew Mew is a transgender alegory (whoops I can see the upvote ratio going down already for saying this). This fits in perfectly of course with the theme of acceptance...and something very notable. Ralsei spends the fight defending the body's personhood and right to do as she wishes. This is the opposite of the previous attitude that Darkners exist to serve lighteners. What better way is there to serve a Lightener than to literally be one's body, right?
But Ralsei doesn't do that. Ralsei defends the body's right to choose, treats her feelings and desires as real and legitimate.
You cannot access Pink's fight without first having gone through the scenes where Ralsei admits to holding things back. You cannot access Pink's fight without the scene where Flowery calls Ralsei 'princess' and hands over cupons for a pink and blue drink, nor without seeing the scene of Ralsei using Fire Magic.
In the fight against Flowery, Ralsei refuses to fight him. I want to repeat that, Ralsei refuses an order from the player. Can you imaine this happening in Chapters 1-4? This whole chapter is about Ralsei learning the fact that the way to defy the prophecy has been within reach whe whole time. Just do it yourself. And...maybe. Just maybe...we actually saw it happen. Why?
"The vast garden is charred in an inferno of jealousy." But...it wasn't, was it? Fire was used against one singular bullet. But the garden was never charred. Sure, Flowery was killed, but the prophecy had nothing to say about that, and neither did Gerson. Of course, we can't say that for certain, because Gerson's story disconnected from the Prophecy more and more as it went on.
But yes. Pink represents both Kris being given a path towards accepting the status quo with the SOUL, and Ralsei accepting the fact that Darkners count as people. The fact that Ralsei can perhaps change fate...including their (using their since the game uses it at a few points) own identity.
This is also why the Weird Route skips the Dark World. There is no accepting what happened.
Anyway, feel free to tell me any corrections or notes you have on this. Please do not attempt to deny a trans reading of Ralsei without first watching through This Video and the parts of it refering to Ralsei. As a trans woman myself, every moment of this chapter with regards to Ralsei made me go "I know exactly what you are doing here, Toby."
Aka. The secret to understanding Pink is trans Ralsei and Kris starting to accept us.