When Arlon Brass attempted the Legendary Great Maneuver, Kalos was weak and he infected Arlon with his doubt.

Whether that led to the failure is not clear. Kalos thinks it did, but Fool tells us that Arlon's partner wasn't qualified. The unnamed partner wasn't able to see Fool, but Arlon told him to lie and pretend he could. So Fool revealed the details of the maneuver, and they tried it, fell, and died.

Kalos' faith as a performer was tested, though, and he doubted. And after Arlon died, Kalos was consumed with guilt. (www.debeste.nu)

Arlon is also Yuri Killian's father. Yuri, in order to avenge his father's death, plots to take Kaleido Stage away from Kalos.