Oskar

Oskar was the son of a Polish peasant family, who 300 years ago gave aid and shelter to Rowena. Over time Rowena grew to love Oskar, and upon learning that he was terminally ill, decided to give the family the gift for their kindness and cured Oskar with a spell, as well as casting a second spell that would take root when Oskar became fully grown, making him immortal.

At some point, he later moved to America and become a Fry Cook in a cafeteria.

Season 10
Crowley approaches Oskar, who's going by the name Seth in a diner and seems like his looking to make a deal with him when he's called away by Sam under the disguise of Dean.

After learned that his mother, Rowena actually falling in love with him and he is the Rowena's true love, Crowley later came to his cafeteria and sedated all of his customer. He later brought him to Rowena.

Rowena, after she met him after 300 years separated, killed him after brief romantic things, with a fatal stab in his neck. In his last breath, she said sorry about what she have done. His blood later collected as the third ingredients needed for remove the Mark of Cain.

Powers and Abilities
Although he never received any training in nor practising any certain kind of magics, due to Rowena's spells, he gained few but incredible powers.
 * Immortality - Due to Rowena's spell, he gained immortality. He could live and keep his appearance even after 300 years passed.
 * Healing - Due to Rowena's spell, he almost instantly recovered from an unspecified terminal illness.
 * Enhanced Growth - Due to Rowena's spell, he become a fully grown full man in a short period.

Weakness

 * Mortality - Although he was immortal, his immortality has certain limits. He still able to died with natural or unnatural causes. It was proven when he was killed by his "lover" Rowena with a fatal stab of a little iron stake in neck.

Appearance

 * Season 10
 * The Prisoner
 * Brother's Keeper