Serenity is purely fictional. The story and characters were made up by the creators for entertainment purposes. There's no connection to any actual events or individuals in reality.
In Naruto fanfic, 'Hassan of Serenity' could be an original character created by the fanfic writer. Usually, such a character is designed to fit into the Naruto universe's existing themes and power systems. It might be a character with special abilities related to serenity, perhaps having some form of unique jutsu or a calm and collected personality that stands out in the often chaotic world of Naruto.
In a romance novel, 'chasing serenity' could refer to a character's pursuit of inner peace. It might be that the main character has had a tumultuous life and is now seeking a state of calm, perhaps through a relationship. For example, a character who has faced many hardships in their career or family life and then meets someone special. This new person brings out a different side in them, and they start to strive for a more serene and content life together.
Isolation is a common theme. In 'dark serenity fanfiction', characters might be isolated either physically, like being alone on a desolate planet, or emotionally, such as being shunned by their own kind due to some dark secret.
In the Naruto fanfic, Hassan of Serenity could possess a range of abilities. Maybe they have a special kekkei genkai that is related to the concept of serenity. For example, they could be able to create illusions of peaceful landscapes that disorient opponents. Or they might have the power to absorb negative emotions and convert them into positive energy that they can use for various purposes, like strengthening their own attacks or healing. Their serenity could also give them an edge in stealth, as they can move quietly and calmly without arousing suspicion.