@zStelly
The literal is "Viruto" or "Wiruto". Japanese Wikipedia claims it is Wirth, Wild, or Wildt, with the most common being the first. Google is the same.
https://ja.wikipedia.org/wiki/%E3%83%B4%E3%82%A3%E3%83%AB%E3%83%88
You're free to offer a better suggestion, but I always go by most common interpretation on Google.
Wiki is just listing People who's family names are transcribed as ウィルト, not semi-random words like this author uses. "Frieren", for example means "being cold". Good luck finding that on Wikipedia. You'd probably have better luck with a japanese jap-ger dictionary. Or having a tts read the japanese and getting a German to transcribe it.
Anyways, "Wild" also is a word, meaning either "wild" or "game animal". Though that would usually be transcribed ウィゥド.
"Wildt" is an old variant of "Wild", "Wirth" is an old variant of "Wirt". Both aren't used outside of names anymore.
"Wirt" is the closest to ウィルト by pronounciation. Though it makes no sense to call a deceased person a host / barkeeper, while "Wild" would somewhat fit for a guy that left home in anger.
@Zhou
I wouldn't say the japs arbitrarily transcribe. At least when it comes to German. Unlike English or French, you can actually write all sounds of the German language in katakana, only that some of those Kana are exclusive to German and thus hardly ever used. Benefits of being their oldest allies, I guess?
It's only when non-Germans try to guess the spelling that things get weird. For example, the German "L"-sound is written as a lowered vocal (ァォェィゥ) in Kana. Yet as the English "L" is written with the "R"-sounds, most people won't know how to read this.