The Gillian kids are four people from my story The Heritage of Humankind. Their names are Kimvekadrei, Kovetri, Kydre and Kinian. Kim and Kovetri's full family name is Gillian-Garkov and Kydre and Kinian's Gillian-Thunder.
The four are human mutants. They are very telepathic, agile, have somewhat enhanced healing and can manipulate electricity up to a point. To outsiders they look pretty much the same except for eye color: Kim's eyes are bluish green, Kovetri's green, Kydre's violet and Kinian's cyan. There are also slight differences in clothing and haircut, though only Kinian can be clearly distinguished that way. More at Veltzeh's drawing style – Gillian kids.

However, usually the most interesting point about the Gillian kids are their genders and genetic makeup.
Kim and Kovetri are identical twins in the sense that they both developed from the same fertilized egg cell. However, they developed into two individuals by extraordinary (some would describe it incomplete or something to that direction) cell division. The original cell had XX chromosomes, one of which contained the SRY-gene usually found in the Y-chromosome. When the cell divided, one of the resulting cells managed to grab both the SRY-gene and the parts of chromosomes that usually make an embryo develop into a female. The other cell was left with practically no instructions for reproductive organs, and this is the cell that eventually became the genderless Kim. The cell that had the SRY-gene became Kovetri, who is a physiological and hormonal male with XX chromosomes.

Similarly, Kydre and Kinian are identical twins, but the extraordinary cell division went slightly differently. The initial cell was similar to the initial cell of Kim and Kovetri, but this time Kydre's original cell retained the information on how to finish developing the female reproductive system. Kinian's original cell was like that of Kovetri, with XX chromosomes, one of them having the SRY-gene in it.

Witness the chart!

             Kim          Kovetri  Kydre   Kinian
Chromosomes  XX           XX       XX      XX
SRY-gene     no           yes      no      yes
Physiology   androgynous  male     female  male
Hormones     weird        male     female  male