Rena

Rena: An alien race from another universe that can string together magics like thread to create phenomena that humans can only dream of.

Sklera: A race born from both Human and Rena.
Or a human, typically young, that was exposed to such strong magic that their body composition changed to be indistinguishable from that of natural-born Sklera.

Vuli: 3/4 Rena, 1/4 Human. Typically not restricted to aptitudes, and often struggle with magic control.
Blisa: 3/4 Human, 1/4 Rena. Typically only able to cast magic in their aptitude, and often struggle with complex, powerful magic.