Hello everyone!
So, to give a small explanation as to what this is...

Basically, last night me and @Kidthatcandostuff came up with this theory relating the Saxophone Shadow character (Which is the thumbnail for this post)
And we might have found something which might be REALLY important to the Secret Boss And here's how:
We all know that Seam talks about a specific item that will be needed for the next Secret Boss, that is being the Shadow Mantle. (As shown below)

Here's what me and Kid thought of:
The Shadow Mantle is related to the Saxophone Guy, who IS a Shadow Element. (As that is in the game)It was also semi-confirmed that they have at least SOME importance to the story, as shown by this tweet from an official Deltarune developer:

https://twitter.com/JeanCanellas/status/1570439520688164864?s=20&t=Kbz1kr_HnNSObmYqmqtQgQWhat
I think this could mean is that the Secret Boss might ALSO Be a Shadow Element, rather than something else.
I have this small design I came up with, to have some similarities to the Shadow Mini-Boss we see on stage, which is, I'm 99% sure, from Chapter 3.

(Don't take this design to heart, it's just a headcanon design I came up with based on the info I've gathered)
I also feel like if there would be a reason for the Chapter 3 Secret Boss to attack you, it would be because it's tired of being nothing but a shadow, and it finally wants to be in the spotlight for once.

Also, the Saxophone Guy and the Chapter 3 Secret Boss might be connected too?
I'm not quite sure about that but if they were I feel like they'd be because they were both in a band of sorts. (Like in the image above)

And that's it! I hope you enjoyed this little theory!!
Again, me and @Kidthatcandostuff both thought of it so don't think that I'm the ONLY one who made this, he was the one who came up with it, I just thought of more info about it.
Goodbye!! ^^

Edit: HE WAS NOT A MINIBOSS, BUT A NORMAL ENEMY

ALTHOUGH WE WERE RIGHT ABOUT THEM BEING A SHADOW ENEMY