He will have a fire sword that he can throw and call it back just like the leviathan axe and ignite it at will. He will also join Hestia's Familia (he can still a knife as his secondary weapon) and how the story goes if he reveals his spartan rage sooner or later depend on you.
So unlike the canon, Bell has some combat experience. So add some techniques of sword fighting and hand to hand combat when it comes to fighting another person. Because I firmly believe that most adventurers are least experience and have less techniques in fighting against a non-monster opponent
Which gives Bell the edge he needs to defeat a person only two leves above him. He will be starting as a level one and don't make him too op just make him fight smart and he should use the spartan rage as his last resort.
He will have some traits as Kratos (Include the trait of Kratos control his anger and his ferocity) and he can still be kind and genarous. Make his flames white and his eyes glow red and he will be at the age of sixteen, seventeen or eighteen.
Also, He will be apart of a sub-human race called the Spartans. Here is the explanation you can use it in the introduction of the story.
In the old age, The Gods of War were bored as they watch the wars everyday. Wanting things interesting. The War Gods answered the mortal prayers by giving them power that would double their Strength, Speed, Agility, Stamina, Endurance and Durability. (Thats all i can give you but you can add it when you accept the challenge and make the Spartans an endangered species and they went into hiding making other people think that they are extinct)
By the way can you guys make him a part time musician where plays a guitar and sing so that he can earn some money if the dungoen is not enough and he could attarct some girls due to his talent and gain other men's jealousy (yeah make it a harem if you want) and its best that Bell is clueless that he is attracting them on.
And I hope you guys are interested.