I think I read on
a Japanese blog that four of the Executioners (besides the one with the mask) were created from entries in a design competition. They even have names:
Knepper (ネッパー) the acid guy
Uosuke (ウオスケ) the fisher guy
Fione (フィオーネ) the flower girl
Kamika (カミカ) the paper girl
From the looks of the latest chapter, "Uosuke" and "Kamika" are confirmed, so it may be safe to say the other names are real too.