Alexion is not a name, it is an Atlantean title. He is neither human, Dark-Hunter or Shade...he is one of those Acheron classifies as "other".
Alexion is Ias, one of the original Dark-Hunters. Acheron tried to save him by giving a stone holding the man's soul to his wife Liora. Ias's soul was lost when she refused to bring him back to life because she was really in love with another man. He became a Shade and was cursed to walk in between worlds until Acheron took pity on him and brought him back into a semi-alive state by using his blood. Alexion must drink Acheron's blood to survive.
He will do anything necessary to protect Acheron and Simi.
Acheron branded Alexion with his mark: a gold sun with three lightening bolts.
As a Shade his senses are muted; his lot is to observe life, not live in it.
He can only be killed by an Atlantean dagger from Apollymi.
He can explode Daimons with a thought.
He lives in Katoteros, but travels to the human realm for 10 days to act as Acheron's judge, jury, and executioner when Dark-Hunters get out of line. He takes his duties seriously, as he doesn't want to condemn anyone unnecessarily to the same fate he experienced as a Shade. No Dark-Hunter retains a memory of Alexion once he has passed judgement and returns to Katoteros. During one of his investigations, he met and fell in love with Danger, who was subsequently killed by Stryker. Acheron also brought her to live with them in Katoteros.
Simi's sister Xirena is bonded to him.