I havent played deltarune chapter three and four yet, and i am DODGING SPOILERS AS BEST I CAN, but int he meantime, heres what i thought the chapter 3 secret boss mght be like?
Basically its a conjoined twin situation with a moon and a sun, but the sun is crazy!!!! {Can you tell i like celestial pairs?}
They were a little kids tv show mascot, akin to Dora the Explorer or the Moose and bird from the old nick jr mascot ads? Kris and Asriel used to watch them growing up, and the moon talks to kris and susie like theyre children and theyre old pals =]
Suns in the most control of the body, but moon uses any control they have to lock sun in their old studio, hidden behind a locked channel that kris susie and ralsie need to find the code for. Once they break in, Moon begs them to help heal sun, which leads to sun fighting them.
Sun and moon have a split face due to their brak in mind and connection, but depending on the route, you can fix it. at the end of their turn, moon makes the last attack sun shoots out GREEN, to heal you!!!
If you ACT to defeat them, their faces get closer! You can eventually help moon calm sun and put them back together, giving moon more control of the body. This gives you the SPLIT FACE MIRROR, Which for 35TP, shield two of your party members for TWO TURNS, at the cost of them not being able to gain TP for those two turns, even if they are defending, or if on kris, grazing.
If you FIGHT to defeat them, the father their faces get from each other, but you ALWAYS kill moon first because they are a third of suns HP, shattering them, and puttign sun in full control. Suns attacks get much harder, and they Fight you with everything they have, and when you win, You get the star shards! The person you have it equipped on hits the enemy thy choose 3 times in one FIGHT turn, for a tighter perfect hit window.
LLike i said, its just a concepts, and IDK how balenced it would be?
But i hope you enjoyed my silly ideas!!! DELTARUNEEEE NOWWWWWWWWWW