I never understood why Japanese writers in general never tried of using a dictionary, or ask a fluent english speaker in general, especially if they know that their products are big enough to make it overseas.
"Ruler" was dumb, but "Shielder" is even more stupid.
In case you're missing my point, the thing is not that "it's not a word = therefore should not be used", it's "using a word that is way off from the intended meaning" or "has a far more obviously common, fitting, or otherwise effective words that could have been used in it's place".
Like how "Ruler" is a word all right, and can actually be a Servant class, but has nothing to do with Jeanne in the slightest, both the Eirei herself nor what she actually does in the story.
"Shielder" can easily be replaced by "Guardian", "Defender", or "Sentinel", which aren't even difficult words, and should already be known by the RPG-headed people of TYPEMOON.
Shielder is a synonym to Guardian, Defender, and Protector, so saying "using a word that is way off from the intended meaning" is nonsensical.
Saying there is more obviously common and fitting words is true, but the reason for choosing this specific word seems obvious given it being derived from the word shield and thus its association with the object shown rather prominently with the character. The other options lack this direct association, which is why this would actually be the most effective word to use.
The word sentinel was a poor recommendation, as that is less associated with defending through blocking something and more defending by keeping watch over.