Okay, I'll give this a try. (Had to separate the bio format as Jason and Etrigan are 2 different entities)
Superhero/villain: Jason Blood
Real Name: Iason/Jason of the Blood
Age: He dates back at least to Camelot, where is he was merged with the demon. This made him immortal, though he could still be killed in human form.
Sexual Orientation: Straight
Base of Operations: Gotham City
Powers/Skills: Martial arts, Ritual casting, Partial transformation and telepathic link to Etrigan, Transformation into Etrigan
Weaknesses: Switching places with the demon sends Jason to hell, and the transformation process takes a toll on his body. Furthermore, his transformation can also be triggered from the outside, by reciting the necessary words of power to him. Furthermore, his long life is part of his curse. Should he and the demon get seperated, he would start aging rapidly.
History: An accomplished historian and demonologist, Jason only recently uncovered the truth about his past. He is in fact one of Merlin's servants, and at the fall of Camelot, Merlin bound his half-brother, the demon Etrigan to him. Etrigan manipulated him troughout the centuries, until Jason became conscious of the demon's existence by regaining some of his memories. When a demon-worshipping cult captured him and forced him to transform, they mistook his recitation of the transforming spell for a curse and killed him halfway through the process. However, since he was half Etrigan when he died, the demon's regenerative powers saved him, what's more, a telepathic link was formed between them. Battling for control, they finally realized that both their interests are served if they cooperate. Because of this 'death', Jason gained the ability to assume a half-transformed state, giving himself a boost of physical powers, and gain a limited regeneration, so he is not entirely dependent on switching places with the demon. With his knowledge of the occult, and some of his memories returned, Blood can also cast magical rituals and wards, with necessary preparation and materials.
Real Name: Etrigan the Demon
Sexual Orientation: Pansexual
Base of Operations: Hell/Gotham city
Powers/Skills: Superior physical qualities, Hellfire, Nigh-invulnerability, Regeneration, Sorcery
Weaknesses: Has all the weaknesses that demons possess: vulnerable to iron and holy magic, can be summoned and bound by powerful spellcasters, can be banished by reciting the according poem.
History: A demon from hell, originally summoned by his half-brother Merlin to aid in the defense of Camelot, Etrigan was bound to the human Jason Blood. After many centuries, he was released onto Earth once more by reciting his summoning poem. He is a Rhyming Demon, a high rank in Hell's armies, and he speaks entirely in rhyme. When Jason was killed switching places with him, their newformed bond gave him a new form of hellfire, one that could not harm those with a good heart, but burned all that was vile. He agreed to work with the human, finding the change of pace interesting (or so he claims), and recieving some measure of freedom to move around on Earth.
Activity: They plot together against the elusive and treacherous Merlin, who cursed them. Also, as part of their agreement, Jason will let Etrigan roam free from time to time, on the conditions he doesn't harm innocents. Still, the demon has a violent nature and as a self-proclaimed 'good guy from hell' hunts down villains, sometimes crossing paths with other heroes (leaving a strange first impression).
Jason on the otherhand, no longer haunted or controlled by an external force, is glad to be able to lead his own life for once, even if it means letting his inner demon loose from time to time.
Known Enemies: Merlin, Morgan LeFay, Klarion the Witch-boy