The Element of Surprise

Thousands of assassins came after Fruity, Klaus's fourth incarnation, but only one managed to impress him. From the tormented world to Valeria and beyond, he faced many assassins, but Axel was different. 

He was also the only one who got close to killing him.

Back then, when they became brothers, Axel told him his strongest ability was the element of surprise. 

The element of surprise is when you never expect something to happen, yet it does, and you can only try to adapt within that millisecond, or else you will die.

Back then, Fruity adapted within that millisecond when Axel attacked him, and yet again, he did that today, but he died.

Of course, Klaus saw it coming, but to lift the spirits of his friends, he intentionally lost his footing when Mason attacked, which gave Aoi the chance to end him before he could recover.