I did a search for セト on a japanese final fantasy site (not sure if I can post the link) and the only thing that showed up was Seto from FFVII. However I also tried to look the net for Seth translitteration in kana (Seth to katakana search with google) and it is written as セス (Sesu) in the first 3 results...
Nevermind, even the japanese wikia lists セト as one of the possible transliterations for Seth, it seems that the more I look into it, the more complicated it gets...