Bloody Scholar engaging SCHOLARLY MODETrevor is a Welsh name that derives from "trefmawr" which means "Big Village". With so many Germanic and Slavic names in the Castlevania canon, the presence of a Welsh name seems rather out of place. However, his name in the Japanese script, Ralph, is more appropriate. Ralph derives from the Nordic "Ráðúlfr" and its later Norman simplification, Radulf. Ráðúlfr means "The Wolf's Counsel". Fittingly, Trevor's incarnation in Lords of Shadow is associated with wolves and later serves as a counselor to Gabriel, befitting his original name.
Ralph/Trevor Belmont has a bit of a naming conundrum in that depending on the translation, his name is entirely different.
...
So, I tend to side with the Japanese and refer to him as Ralph as (given his point in history) being given a Welsh name just doesn't make historical sense to me, whereas a name of Norse/Norman derivation makes more sense for a character descended from a French family (the Normans being the French descendants of Viking conquerors under the command of the warrior-king Rollo, the best-known ancestor of William the Conqueror, probably the most famous Norman of all time.
...
That being said, I always thought "Ralph" sounded silly when spoken out loud, and so in casual conversation, I usually use "Trevor" -- it sounds cooler, and American fans generally understand who I am referring to more quickly.
Fernandes/z is also linguisitically appropriate, being a common surname in the former Gaulic countries (Spain/France/Portugal), but the Belnades family hails from the wrong part of the world (being principally Slavic). The name is most likely
meant to be "
Vernandes" which is a real surname that can be found across Europe in a much wider range and is more Slavic-appropriate. The sticker came when both names are rendered in Japanese as (roughly) "Verunandesu" from which the translators somehow got "Belnades". It's far from the most glaring goof of the series though -- "Belnades" at least sounds somewhat regionally appropriate to the Wallachian setting of the early Castlevania games.
As the Wiki points out,
The sound of "B" is similar to "V", while in some languages like German, "V" is pronounced as "F".
Likewise, "S" is pronounced as "Z" in some languages, such as German.
The sounds of "-el" and "-er" are usually translated into "-エル" (-eru) in Japanese.
It's also worth noting the original American release of Castlevania 3 got Sypha's name as "Syfa Velnumdes", which is even more off the mark than "Belnades".

The historian in me is compelled to point out though that in a way,
Fernandez and
Vernandes/Belnades can both be canon if said individual has Belnades ancestry due to a Belnades marriage with a family named Fernandez. Due to patronymics and other good stuff, said person would likely be known as "Fernandez" after that.
Or, more simply: all Fernandez's in the story are Belnades', but not all Belnades' must be Fernandez's. Zzzssszz.
EDIT SO THIS IS MORE ON TOPICI generally go for whichever version is more accurate to history and narrative intent. Orlock instead of Olrox, Cthulhu instead of Malachi, Ralph instead of Trevor, etc.