There's nothing personal about defining a waifu as a non-existent partner who unconditionally loves, desires, or is attracted to the main character. Nothing personal at all. I don't see the point of using an overly broad definition of the term when I'm only interested in a single aspect of it for my argument.
Think of it like this: I say "If we need to reach the North Pole, we should take a plane to fly across the ocean". Then you respond "That vehicle where people sit in chairs one after another in two rows? We'll never reach the North Pole on a bus" So I reply that you incorrectly understand what plane means. Then you and Taril argue that people also eat, sleep, and use the bathroom on planes - so claiming that planes fly over oceans is just my personal, narrow definition. But I don't care about what else planes do. I'm talking about the concept of reaching the North Pole - that's it. And I define a plane as the means best suited for that purpose. Abstract thinking, you know? Same with waifu - I don't care about the million other things people associate with the term. I want one specific trait - unconditional loyalty. No porn. No fanservice. Just a female companion with pure, unwavering devotion - or simply a "waifu" for fck's sake, to avoid writing a hundred descriptive words.
Except the issue with that is the VAST majority of uses have ABSOLUTELY nothing to do with your one specific definition.
To the point where the actual basis for the term has no regard for them whatsoever.
Meaning you are adjusting the basic and most understood meaning of the word to mean your personal version of what the word means and then assuming that everyone will simply understand that context, despite how niche and obscure using the term in this way actually is.
It's like saying:
"I would like a car"
Everyone would think you mean some form of automobile for personal transport on a road and then going:
"NOOOO I mean a train car, a vessel for transporting passengers or goods on a rail system that is powered by a train, gosh it's SO obvious and very clear I meant this very specific definition and nothing else"
Also... You manage to sum up your meaning in a whopping 7 words. "A female companion with pure, unwavering devotion"
How is that "Writing a hundred descriptive words" and necessitate the use of an niche and obscure meaning of a word as replacement? I suppose you can throw in "That I find personally attractive" for an extra 5 words and to show why Karlach doesn't count. Still... Hardly "A hundred descriptive words" and it avoids any confusion with general meanings of terminology.
IF you still wanted to use some abbreviation you could simply use the term "Wife Material" which is more inline with your usage of "Waifu" as it is specifically about someone who is considered ideal to be ones wife and has absolutely no connections towards sexual desire and is purely about "I would like a long term relationship with that person"
Of course, with this being more generic, the statement that 'Shadowheart, Lae'zel and Karlach are not "Wife Material"' is still not true, as many people do find these characters to be suitable for their personal definitions of "Wife Material" - Hence why actually explaining your personal view of what you want a character to be works better for your statement, saying "Shadowheart and Lae'zel do not show pure, unwavering devotion" is factual and not an opinion. That you think this fact is a problem, is then an opinion backed by facts.
People may then disagree with your opinion, but they cannot disagree with the basis for it as the basis is fact. As opposed to terms like "Waifu" and "Wife Material" which are exclusively determined by subjective opinions and thus can never be fact.
