Postscript: Turned out to be available in the updated event with the June 6th update. For details, see the link below.
I noticed that I was updating the title list, but new titles [Extreme Showdown: Super Warrior] and [Extreme Showdown: God] have been added. The place to obtain these two titles is the event "Extreme Showdown! God and Super Warrior", but you can't find the way to get them even if you look at the event...
I thought that the event "Extreme Showdown! God and the Super Warrior" would be completed in 4 episodes, but it will be added in the future, and I wonder if the clear rewards are these two titles...