Note: This is advice on how to create a cool Keshin name, not a law.
When thinking or creating a Keshin name, a few points have to be taken into account.
> The name in Japanese is Kanji-Katakana (Except Tenkuu no Shihaisha Houou)
So if we use Majin Pegasus Arc as an example:
> 魔神 ペガサス アーク is the same as マジンペガサス アーク but the Kanji-Katakana looks so much better and professional.
> This must be taken into acount if you make a Keshin as you may want it to look/sound as professional as possible.
So if you make up a keshin, remember to translate it into Japanese using Google Translate or other English-to-Japanese websites. When using Google Translate, click the Japanese and a mini-menu with alternatives will appear. Make sure the 1st part of the Keshin's name (eg: Majin) is in Kanji, the rest can be in Hiragana or Katakana.
Have fun making Keshin's