Armin Arlert is the standard romanization I've seen for his name, including the official Kodansha spellings, though a few places have used "Arlelt" instead. Since the katakana for his name is given as "アルミン・アルレルト" (Arumin Arureruto), the current "Arelet" is not very accurate at the very least.
Updated by jxh2154