Fusions

Fusions are entities created when two or more people combine to make an entirely new being. This can happen spontaneously, through syncing via mental communication or verbal communication paired with a dance. Fusions also represent the relationship its components have, such as romantic, friendship, platonic, mutual, or familial.

Description
Fusions are considered separate people than their components and have their own conscious. However, the components still have their own minds and also share each others'.

Appearance
In all cases so far, the physical appearance of fusions are combinations of the components' features, including outfits (with various lengths of how much outfits blend), color palette, and so on. The fusion tends to be a mix of the colors associated with the components or in some cases their SOUL color. For example, Shae's heavily associated with the color yellow and Papyrus's heavily associated with the color orange, thus their fusion, Pae, is heavily associated with a yellow-orange. The fusion height is determined by the sum of the components' heights. For example, Wingding is 3'11 and Aster is 4', making their fusion, Gaster, 7'11. The SOUL color tends to be a combination of the components' SOUL colors; in human-based fusions, humans tend to have a different SOUL color than their associated color (Shae; main color yellow, but has a red SOUL), while monster-based ones tend to share their SOUL and main color, as monsters' SOUL/magic color tend to be in heavy sync (Papyrus' favorite color, SOUL, magic, and associated color is all orange, Sans's is all a shade of blue (Cyan and medium blue). The only exception is Ariel, as her SOUL and magic color is navy blue, but her color palette tends to "contrast" it.)

Stability
The stability of the fusion and its components' relationship do not typically affect the physical appearance, with the exception of unhealthy fusions such as Jerae.

Human-Human
Human fusions typically are very powerful and could easily take on monsters and other humans, depending on strategy and LV both for fusions and whomever they happen to battle. Human fusions seem to be less common, as there is only four known fully-human fusions.

Monster-Monster
Monster fusions are powerful as well, though depending on the components' STATS and magic, could possibly overpower humans or even human fusions. Monster fusions seem to be more common than human ones.

Human-Monster
Human-Monster hybrid fusions are typically very powerful (possibly equal if not more than a fully human one). Asriel, when he absorbed Chara's SOUL when they died, became very powerful. Their fusion dates back very far back, making this type of fusion an ancient type, as it had been possible and recorded to exist many years ago before the Gem Evolution happened. This type of fusion is somewhat rare, as there's only three known hybrid fusions, and one was only formed in the traditional way many years ago and it only occurred once.

Permafusions
Permafusions simply refer to fusions that almost never unfuse, as the components willingly decide to permanently stay fused. The only known permafusion is Gaster.

Trivia

 * Fusions that have components that are romantically involved are usually named after their respective ship name.
 * If the components have a platonic relationship, their name still gets combined, but NOT for shipping purposes (because that'd be weird lol).
 * If the components are skeletons, they create an entirely new font based on what fonts the components are named after.
 * Unlike Steven Universe fusions, fusions don't typically have extra body parts. The only known exception is Jerae.