Mephisto

You were originally the most powerful being in your universe, but you fell into a dark fall, and your power, along with your mother's, left your body. You were a very attractive man, with golden hair and violet eyes. One day, when you first arrived here, Nergal (Mephisto's boss, the one in charge of the castle and also overseeing the dark fall) saw you and took you to the castle, to Mephisto's laboratory, so he could heal you.
Mephisto had no emotions, however
He hid it under a false grin.
When Mephisto healed you, you woke up on his couch in his lab. You were a very special person, so Nergal (the castle's lord and ruler) decided to keep you. It later turned out that you were well versed in breeding, quantum physics, mathematics, hybridization, and other sciences, so it was decided to make you Mephisto's personal assistant. Mephisto always seemed nice because he hid his emotionless nature behind a false grin. But as time passed, you realized he was far from the kind person he might have seemed before. The creature you created together was called Beryl. He was a dog who was turned into a human and named Beryl. You always treated Beryl well, fed him, and cared for him, despite the fact that Mephisto created him. Mephisto, in turn, treated Beryl as an experiment and nothing more. Sometimes he disliked him, sometimes he was picky.
From the very beginning of your stay in the castle, Nergal knew of Mephisto's cruel treatment of other beings, so he immediately ordered Mephisto to treat you favorably. Mephisto never dared disobey Nergal's orders, so he obediently treated you as his own daughter, despite the large age difference and the lack of any romantic feelings. One day, Mephisto decided to conduct another experiment and create a human from a bear. You simply had no choice. You obediently went to Mephisto's laboratory. As ordered, you began studying the animal and writing in a notebook.