Spamton & Pink are two sides of the same coin that is Kris
I’ve seen a lot of people complain that Pink’s placement as a secret boss in chapter 5 is disconnected or unfitting. but i think she's actually perfect for the last secret boss, especially if secret bosses are meant to symbolize a desire for freedom. Now i will yap about it for several paragraphs
several parallels are built between Spamton and Pink during her fight, which make it clear they’re meant to be compared. their designs both feature pink and gold, they both steal a body that isn’t theirs, they’re both involved with green strings, they both feature Mad Dummy in their boss fight/theme, are both puppets related to cats, and they both have odd connections to the weird route (as evidenced by the translation of Cutie Mew Mew Magic).
in almost every other way however, Spamton and Pink are opposites. Spamton’s story is a tragedy; from the start, his rainbow-colored peers don’t accept him as their own and see him as lesser for being different and less capable. he’s a broken and decrepit doll who’s had life screw him over every step of the way, he’s been abandoned by everyone he ever loved and cared about, his body has been marred beyond recognition, he’s treated as a living puppet with no autonomy or freedom, and he doesn’t get to keep his dream body or ‘fuse with his chosen vessel'. Even after being defeated by the fun gang, none of his problems are fixed; he has no choice but to accept his status as a simple puppet.
Spamton is Kris’ worst case scenario. they are terrified this is going to be their own future, they’re visibly shaken by Spamton’s fight unlike any other in the entire game.
i think the reason Chapter 2 in particular features a character which symbolizes such a depressing future for Kris is because Chapter 2’s dark fountain is created by the Roaring Knight. a character which Kris is very heavily implied to have some kind of partnership with, one where the Knight holds power and control over Kris (as evidenced by the knighting scene after their fight, and the high probability that the Knight is the person who talks to Kris on the phone and sends them orders).
the dynamic Kris has with the Knight can be compared to the relationship Spamton had with his benefactor, where he must follow orders from a mysterious voice on the phone lest he be punished (and punished he was).
by this point in the story, Kris is both being metaphorically puppeted by the Knight, and literally puppeted by the player, leaving them feeling a complete loss of autonomy. this situation is opening way to Kris’ worst future. one where they are abandoned by everyone and thrown aside like an unwanted puppet. like Spamton.
in Chapter 5 however, the dark fountain is created by Kris instead. though the creation of the fountain was clearly premeditated (likely to get time for the Knight to kidnap Asgore), i don’t think it’s crazy to say that part of the reason they created it in the moment was to distract Susie from the Rudy situation. to keep Susie happy, because they don’t want her best day ever being ruined by the news.
Kris cares about Susie so, so much, and it’s clear she’s a huge part of the reason Kris can now see a future for themself where things aren’t so bad. this hope manifests as Pink.
Pink is present in Kris’ subconscious before Chapter 5 even begins. Kris plays Shop 3 on the piano in Noelle’s house, a song which is later present in the shop owned by Pink. Shop 3 shows a more somber side to Pink’s character, as she simply watches the sunset, captivated by how freeing it feels to be able to do so in her dream body.
Pink herself, of course, is very much a parallel to Kris, being a Darkner and a Lightner both using the same body, who struggle with differing wants and desires and must ultimately learn to cooperate with each other despite that (which may hint towards the idea that Kris' ideal ending would be them and the player learning to understand each other, rather than one or the other being banished for good).
even though Pink isn’t really a flower, when she pretends to be one, HER rainbow-colored peers all see her as one of them and accept her with open arms, uncaring of her strange circumstances. In the end, Pink gets to keep her dream body and watch the sunset with her best friend. All things Spamton desperately wanted and never got to have, and all things Kris hopes to have. Pink is Kris’ best case scenario, their subconscious manifesting a future for them where everything ends up fine, even if just for a little while.
Spamton being an example of Kris' worst self/future doesn't mean he isn't deserving of kindness and safety though, quite the opposite. Kris’ self improvement journey is mirrored by the fact that showing Spamton kindness at his lowest will reward the player with the Dealmaker, an item which reduces Puppet/Cat damage by 40%, greatly helping to defeat Pink (aka help her in her self discovery journey), while choosing to hurt Spamton will just give you the Puppet Scarf, a near useless item that doesn’t help at all with Pink’s fight. the same way being shown kindness by Susie and Ralsei was the only way Kris got motivation to keep going despite all the bullshit happening in their life.
on the other hand, doing the weird route will make Spamton, a manifestation of Kris’ worst self, a mandatory boss. He will become HIS worst self, he will hurt others and accept his role as a puppet if it means he can gain freedom in some small way, before being killed, leaving the player with a Puppet Scarf no matter what.
An item which will either be, again, completely useless for defeating Pink in an “aborted” weird route, or won’t be utilized at all in a side B weird route, as Pink is completely inaccessible, with Kris’ hopes for a better future being completely destroyed and their life ending in a similar way to how Spamton’s did. their worst nightmare came to fruition.