[Annuga]

Toei posted some details, don't think these were mentioned

The hero’s name was coined from Extreme and Aid. A Kamen Rider who must save the world from certain doom, and a Kamen Rider who saves the lives of his patients - that’s Ex-Aid, “The most extreme of aid”.
X-Sports (Extreme Sports) have become an influence, particularly the letter X. They remark that the letter X is the Roman numeral for 10, and that Ex-Aid is “A person who must clear 10 kinds of games”.
Ex-Aid is not only about changing the fate of patients whose lives are on the verge of death, it’s also about changing your own fate and those who hold the world in their hands.