Wikipedia listed her name is "Mathilde", which name is true?

A little googling I found this:
MATHILDE
Gender: Feminine
Usage: French, German
Pronounced: ma-TEELD [key]
French and German form of MATILDA

So it is very possible that Mathilde was the intent, they just, for some reason, adapted it to kana in the same way one would adapt the name Matilda, though something pronounced ma-teeld would (probably) be better written as マティルド.

According to other sources, it is pronounced more like it is written, which would then more likely be マスィルデ.

tl:dr
Basically, whatever it is, it's Matilda or a varient thereof.

Who cares. I want more pics

Thread very relevant to my interests.

Here's an eye catch screen cap from Shakugan no shana II episode 12