Idia self-identifies as an otaku in-game, and he talks similarly otaku from years ago who were infamous for using honorifics like “-shi” and the word, “degozaru,” as Idia does.
The Japanese language has many pronouns, and Idia switches between two of them: He uses “sessha” in general and will often shift to “boku” mid-conversation, usually when he starts to panic.
“Sessha” used to be mainly used by samurai, but it was adopted by people who identify as otaku several years ago.
If you are an anime fan, you might be familiar with the word “desu.” In addition to being required grammatically it can also be a politeness marker, and there are a few variations depending on how polite you want to be.
To rank them: “neko-da” is informal, “neko-desu” is more polite, and “neko-degozaimasu” is formal. They all mean, “it’s a cat.” They do not change meanings, just formality.
“Degozaru” is something of an outlier and is not used in normal conversation. Once used by samurai, it was adopted by self-described otaku a while back.
If you search for “why do otaku use ‘-shi,’ ‘sessha,’ etc” (and it seems to be a not uncommon question) one of the best answers available is, “because it’s fun.”
While technically Idia is adopting archaic speech patterns he is also just speaking in outdated otaku/net slang, which is also how he was localized for EN.
If you are a language student and worried by his dialogue, don’t be! He is written to be unintelligible, with JP-server players also commenting that they cannot understand him and even his own voice actor Uchiyama Kouki saying that he sometimes can’t understand his own lines.
Twisted Wonderland Reference Note 