It is believed that there are no consistent patterns existing with the Shadow Crystal/Secret Bosses in the DELTARUNE. However, with the release of Chapter 5, this rhetoric have been proved false. There is one consistent pattern, a rule by which all of these bosses are set, that is the alignment with Circles of Hell from Dante's Inferno.
If you're not familiar with that theory of mine, then I'll repeat it here.
Each shadow crystal boss of the DELTARUNE in each chapter is aligned with a Circle of Hell from Dante's Inferno, like this:
Chapter 1 - Treachery. The Shadow Crystal Boss is a "Devil"
Chapter 2 - Fraud. Fits the most with Spamton, it's also implying that he is perhaps copying somebody?
Chapter 3 - Violence. Roaring Knight is a Chara boss added into an official game, and the Mantle Minigame calls back to UNDERTALE's genocide route.
Chapter 4 - Heresy. Gerson is a heretic, he goes against the DELTARUNE prophecy. Also this connection of a religious theme between Dante's Inferno and DELTARUNE is quite suspicious, isn't it?
Chapter 5 - Anger. Mad mew mew boss fight all again. The Boss doesn't betray us, nor tries to trick us, nor tries to harm us, nor tries to manipulate us into accepting heresy. She just beats us up or out anger.
Now, you might say that there are only 7 Chapters not 9. But I think there is a good reason for that.
Because Chapter 7 would align with Gluttony. The word Gaster literally translates from latin as belly or stomach. It draws an immediate connection to Gaster being a final boss, but also to the Weird Route of Chapter 5 in which two things are revealed: that Chapter 7 will take place in Depths; and that songs from UNDERTALE sound test fun value room are going to appear, along with Gaster's Theme, as Happy Town plays in the overworld on the route.
Then, Chapter 6 would align with Greed. I believe that it would be Friend. Why you might ask? Firstly, because of the DogDollar item, value of which decreases with every chapter and that if fused with GoldWidow leaves you with no money after a battle - this implies that a reversed version of this item, "CatDollar", would have a huge price. Secondly, the new Friend dialogue implies that Friends are "searching for protein" and would "chase the target until they're bored", and that some people would go hiding to avoid them, alluding to Spamton. Spamton not only have gone hiding, but also lost all his money and lived in a dumpster, which could imply that Friends can find you by sniffing out wealth.
These cats are greedy and gluttonous is what I can conclude. Thus it makes the most sense to me if Greed boss is indeed Friend. Since there are no Shadow Crystals beyond Chapter 5, and Gaster seems to be a major boss of Chapter 7, I can make a speculation that Friend is the major boss of Chapter 6.
You might say "UMM YOU ACTUALLY SAID THAT FORGOTTEN MAN IS A CAT", and uhhhh that is too a possibility, because well... If you don't need to "look for the man" anymore, it could be that because he will come to you. "They'll see the Tail of Hell take crawl", after all.