... for the first example. Wow.
Desu is not necessary at all, and the idea of someone including that after
every one of their sentences tells me they either don't know
what "desu" means or they think that it's cute.
"Desu" is not one of those pieces of Japanese that are acceptable in English fanfiction.
However, I have to agree with the deviant I originally responded to in regards to incantations, suffixes, etc. It may or may not be because I am very partial towards Japanese honorifics if they are used correctly. I am actually guilty of the usage of those in quite a few of my fanfiction, and after a few years weaving in and out of the fanfiction community I've learnt what looks and sounds pleasant in a sentence and what doesn't, and I think that's what I base my writing upon most. However, in an original (perhaps published) story, I would never include the use of those suffixes unless the situation drastically calls for them.
But because this is anime and most of the fanfiction I read using said suffixes gives me the proper atmosphere to build upon in that world (such as Bleach and Death Note, both of which take place in Japan), I will often approve such use of honorifics. However, I will not approve of such honorifics in fanfiction for Black Butler, Hetalia, etc. Since those should be primarily English-based (BB is in England, Hetalia is international). As quoted from the PPC wikia:
"Japanese honorifics are one of the few pieces of Japanese that are acceptable for use in English language fics, as long as they are used correctly. If you can't use them correctly, or are not sure of how a particular honorific is used, then you should leave them out."
This is how I personally feel.