Efil’s curse was actually a high-level fire-based seal. Kelvin’s ability to remove it so easily is another hint at just how broken his "Summoner" rank and Mana pool are.
Kelvin visits a slave market to find a companion who can handle chores and assist in combat. He meets Efil , a Half-Elf with a "Cursed" status (a permanent burn on her back). Kuro no Shoukanshi Episode 3
Kelvin reaches Rank D. Because his actual stats are monstrously high for this rank, he has to hide his "Summoner" class to avoid unwanted attention from the Kingdom or the Goddess’s enemies. Efil’s curse was actually a high-level fire-based seal