Deban, Warla and Blessing all do the exact same thing; they add the caster's mental score to the party's base defense. This is why Blessing gives the biggest defensive boost most of the time while Deban gives the smallest and Warla is somewhere in between. Raja's mental stat is only rivalled by Rune's, while Rika's mental stat is very low. This also means that if you cast Blessing first, then Deban, your defense will go down...Blessing's big defense boost is replaced by Deban's okay defense boost.
And Rayblade is actually Nathu in skill form, with a slightly higher base damage. The difference is minimal, but since Chaz doesn't get a lot of tp and Nathu is pretty expensive, I use Rayblade a lot more than Nathu.