actually, all of them lose the memory about the six finger in Walter park. It was necessary and it's da magic
Which guy? Iruma lost his memories just likes the other because the power of one of the six fingers member. If u asked about which arc its amusements park arc (which honestly makes me kinda aware we have a lot amusement park kinda moments in the story).
Also, if you're talking the small guy he's also a member of da six finger. But if you're talking about the guy in the last panel, apparently he's kalego big brother who hates iruma because he is human.
When did Iruma loose his memories of her? And who is the guy again?