Okay, so firstly I'm sorry if this is too memey of an idea. I'm no theorist and frankly I don't know who the Knight is, but I feel that any character we've seen (besides MAYBE Rudy?) could carry Undyne or Asgore in the light world with no issue, and with no special Dark World shenanigans, which I see argued isn't possible for some Knight candidates. So I will try to reason why this isn't a problem.

We see at the end of Chapter 2 that Kris gathers up everything in the computer lab. They don't carry it like the trash orb in the beginning of the chapter, but they have to be carrying it somehow (and not traditionally as an item we most likely would have seen a "Laptop" item in the inventory, could be like what lets them hide their knife and Berdly's feather from us). Well, why can't the Knight do something similar? It seems reasonable that a person (even a heavy one like Asgore) is much lighter and smaller than a whole computer lab right? And if a scrawny teen like Kris can carry all of this, then the Knight should be able to carry something similar in weight. Even if said person is fighting and kicking that's arguably still easier than carrying something way larger and way heavier. Not easy, just possible. Even if they didn't take the whole computer lab, they'd have to at least take many of the items like the trash can, laptop + computers, mice, power strips, all of which can be carried without the player seeing, so couldn't the Knight be able to carry people without anyone else seeing as well?

Furthermore, we see Susie able to balance a CRT on her head and walk no problem. Now I don't know how heavy Tenna is, but CRTs are typically REALLY heavy. The weight of all that glass adds up, and some larger models can be heavier than a person. Judging by its size compared to Kris and Susie, I'd say it's likely around 50 pounds, maybe heavier (i'm not sure how to accurately measure the weight/size of Tenna so someone can chime in if they have a better idea). So all that to say, if Kris and Susie can carry heavy things without struggle, the Knight lifting people is probably not an issue!

Let me know your thoughts because this could be totally off-base, but might be fun to discuss I think! I guess this assumes the Knight is a Lightner that has to follow Light World rules, which could be totally wrong to begin with!

Why is the Knight needing to be able to carry people an issue when Kris can carry an entire computer lab in their back pocket no problem?