Engineer gave them powers from "Quantumtale"
which makes SMG4Tale characters be able to use Undertale AU fusions
Melony can use "Sans" and "Frisk" fusions
Meggy can use "Undyne" and "Chara" fusions
SMG3 and Mr. Puzzles can use "Gaster" fusions
SMG4 can use "Sans" fusions (only at level of Underverse/Sagaverse) and "Asgore" fusions
Tari can use "Papyrus", "Toriel" and "Alphys" fusions
Saiko can use "Mettaton" fusions
bob and fishy boopkins can use "flowey" fusions
but Saiko, bob and fish boopkins don't use thier fusions often