User blog comment:B214/High School DxD EX/@comment-178.128.187.173-20150903045503/@comment-26535460-20150904211131

The various forms of his name come from the direct reading of the katakana that form his name "Ikusu". When translating it into English, this can end up pronounced as either "Iks" or "Ikus". "Iks" then becomes Ix while "Ikus" has some how been changed to Ixis even though we would read it as "Iksus". Ex is simply an alternate translation of "Iks" suggested more recently for various reasons, including the fact that it matches the "EX" from the title. In fact, at one point it was rendered as "Ichs" as well. Anyways, the simple fact is that we do not know how its supposed to be, and are only making an educated guess.