Mentor

Mentor is Sha-ra's twin brother and rival. He's strikingly similar to Sha-ra, not just in apperance, but in personality (he's more like an angry Sha-ra), powers, traits, and issues. While Sha-ra blames Mentor for the fires that first killed their mother, then their teacher, Mentor blames him. He and Sha-ra also mirrors Alk and Ilke rather well, by Sha-ra and Alk being responsible, while Mentor and Ilke being quite wild often not taking responsibility.

Tarkan
Mentor is the current host of the evil thought, Tarkan, travelling around Aldra following Sherpa's orders. He manipulates people if he has a chance to and tries stopping Alk and Ilke. He's fuelled by his anger towards Sha-ra as well as Prince Tarkan's wish for revenge on King Veha.

Amnesia
After King Veha forces Tarkan to leave him, Mentor is left a scared child in an adult body not remembering anything. During Mørkets Barn he tries to recover his memories while also trying to keep Tarkan away from possessing him again as he know that would be dangerous as well as staying away from people so Tarkan can't posses them either.

Same person but split up
Book 8 reveals that Mentor was actually one of Sha-ra's hallucinations growing up with schizophrenia. His parents treated this imaginary friend as nothing serious until Sha-ra set his parents house ablaze with his mother dying inside. Ideus then sailed across the ocean to Arkin to raise the boy under King Veha's orders.

After restoring his memories he decides to live as Sha-ra.

Theories
A theory is that Sha-ra absorbed a twin in the womb, also known as Vanishing Twin Syndrome, overhearing his parents talking, his guilt may have caused him getting schizophrenia imagining the twin to have survived.