I was reading a manga in which the furigana for 僕 was ぺット (pet), this kanji has many readings such as やつこらま (slave, server, etc…), しもべ（servant; manservant; menial）and of course, ぼく (the last meaning in Jisho for ぼく is manservant). So “pet” when referring to a person may fit well the meanings of slave/servant. But the reading ぺット does not appear in any dictionary. Attaching new readings to a kanji only occurs in manga/light novels or also in Japanese literature as well?
This sort of use for furigana is not so much attaching new readings to kanji, but rather adding a meaning to the reading - the furigana is what they say, but the kanji is what they mean. Which is to say, you’re not trying to ruffle through the possible meanings for 僕 to find one that might be interpreted as synonymous for “pet” - it’s just the standard “I”. The speaker is calling themselves “pet” in place of the first-person pronoun.
The Aria manga makes frequent use of this. For example, the Earth is referred to with 地球 - which is, the people of the time now call Earth “Manhome”, but for the reader’s benefit, it’s also being explained that what they’re referring to is the Earth.
I know the context wasn’t in this post (it’s here), but in all likelihood the 僕 in this case isn’t “I” but is one of the “servant” meanings.
Yeah, it’s normal to define words with kanji and introduce the pronunciation with furigana.
In Dune (which yes, is originally English, but still), there’s a fictional instrument called a baliset.
In the story, it first shows up as 九弦楽器. This tells a reader everything they need to know. They know the actual word is バリセット, and they know that it’s a 9-stringed musical instrument. All without having to devote a sentence to that.
In addition to what others have said, another example where I’ve seen this done a lot is 極主夫道, where the normal writing indicates what the main character means to say (which is usually quite friendly) and the furigana give the yakuza slang he actually uses (which is… decidedly less friendly )
So you’ll see things like 包丁 when he’s talking to a kitchen knife salesman or 商品 when he’s bargain hunting - so we as the readers know what he intends to say, but we also know what the other characters are responding to. I found that a fairly interesting way of using furigana
In a slightly different usage there’s even another instance where the knife salesman says 年寄り basically specifying what he means specifically by カモ.
I guess the key takeaway is that furigana always tell you how to read/say something, but that doesn’t always mean they give you the reading for whatever kanji are used, the two can be completely unrelated to one another in theory.
Completely unrelated: your post just reminded me to try to make hamburger patties by mixing meat with fish tossed in the food processor to add smoothness…