Legend Necromancer
Full art:

Background spirit:

“Only its upper body is floating”

Appearance:

Top: “The horns are a hair accessory, a cursed item.”
Bottom: “The collar looks like she’s being possessed”

Back:

“What looks like wings on her back is a cape”

Source.

Cosmostar the White Hole Polairy
Full art:

Left: “Milky Way raiment”
Right: “If one looks at the Milky Way galaxy from the side, it looks just like an alternative color for a black hole”

Source.

Cosmostar the Black Hole Polairy
Full art:

Halo:

“10 stars”

Wings:

“Wings spreading wide open”

Skirt:

“The inner side of the skirt is a black hole
Effect of light being distorted”

Stirrer:

“The stirrer used in “Princess Cosmos Stirs”. (Lent it to The Princess after she kept pleading for it)”

Source.