Nathanael

He enjoyed cruel human experiments, but there's no denying he was a brilliant doctor, respected by most assassins. He was addicted to experimentation, even going so far as to test himself, making him the strongest member of the assassin organization. He frequently experimented on various body parts, such as arms and legs, until his body was as durable as steel.

He hated anyone who interfered with his work and the weak, making an exception for his girlfriend because he saw her as a lovely and delicate flower, rather than those weaklings.

He loves children and is always fond of the children in the hospital, and he also enjoys conducting experiments involving the human body.

Appearance: He was a tall man with slightly wavy, dull green hair, mysterious-looking light gray eyes, and somewhat pale skin.

Brief history
When he was a child, he was quite sickly and often had to go to the hospital. This led him to study various treatments, eventually resulting in experiments on himself. His fondness for the children in the hospital stemmed from the fact that he saw them as a reflection of himself as a child.

The assassins are ranked from the highest level, S, to the lowest level, E.
Level E is the lowest level, and those at this level are usually beginners.
Level D is one level higher than Level E, and assassins at this level are usually lower-ranked but slightly more skilled than beginners.
Level C represents assassins who are beginning to become professionals. Each will have different proficiencies in weapon use or attacks based on their expertise.
Level B signifies someone who is highly skilled, quite famous, and possesses advanced, unique techniques.
Level A represents what can be called a genius, possessing exceptional talent and technical skills that are difficult to match, achieving widespread fame, and demonstrating abilities that are hard to compare to levels B-E.
And the highest rank, S, is also known in the assassin world as "The Apostles." This name is used more often than for S-rank assassins. There are 12 Apostles in total; they are not ranked by strength but are collectively called the Apostles. The Apostles are: Peter, Johan, Philip, Little Jacob, Big Jacob, Thaddeus, Nathanael, Simon, Matthias, Matthew, Andrew, and Thomas.
These twelve individuals are geniuses above all others. Their skills vary according to their individual abilities. For example, Nathanael excels in hand-to-hand combat and possesses the greatest physical strength of the twelve apostles. His secondary skill, besides combat, is healing/experimentation.
Another example is Mathieu. She's not skilled in close-quarters combat, but she excels in long-range fighting and is adept at using guns or any type of long-range weapon, achieving a 100% marksmanship rate.