Boogie's appearance matches that of Flowey from Undertale, only that their color is much more faded, along with droopy low-hanging petals, giving them a much more depressing feeling.
In in the beginning of the RUINS, Boogie decides as the first thing when the human shows up UNDERGROUND is to teach them about where they are. They tell the human of their red soul, being their essence, along with telling them about LV standing for love in a scene paralleling the original. But given his former self is Napstablook, his introduction gives off a more gloomy and depressing feeling due to the lack of Boogie changing expression, and the lack of enthusiasm when teaching the human.
Boogie shoots one bullet, being surprised at how much damage it had done when the human ran into it, but chose to let them go. Despite this, Papyrus ends up hitting them with a bone attack, causing Boogie to be thrown off the scene.
At the end of RUINS, Boogie tells the human about it being nice that they able to carve their own path and being able to play by their own rules. They also told the human about the monsters being nice and that they were happy that they weren't killed. But then asks the player, "But what if someone comes after you with a strong urge to kill you?" and states that "It's not like everyone is a pushover like Papyrus". They tell the player they think it would be possible for them to just easily trample everyone's hopes and dreams, just like the original Flowey. But it's done in a way that's less threatening and much more apathetic.
Coming soon, I guess.
Also coming soon. Maybe.