KICKS OPEN THE DOOR!! I see!! That post about RTK powers and LET ME TELL YOU THAT YES KAY IS ABSOLUTELY A MAGIC ANIME KNIGHT!! He’s even called out in one document as being one of Arthur’s “Enchanted Knights”, which is a pretty cool club if you ask me.
Now, I will note that these powers are normally attributed to Cei, which is the Welsh name for Kay. As the mythology continued to be told and retold over the centuries, finally written down, and ultimately translated into the different languages a lot of these abilities got lost or completely removed from “Kay” as we know him today. But that’s not the point.
As mentioned he can go 9 days and 9 nights without breathing... and without sleeping. Yeah op didn’t mention that one! It was quite a useful ability for him as he used it in a rescue missing during Culhwch and Olwen, and then I believe in a different documents to save the butts of some of his fellow knights.
As mentioned he is a human space heater, he is able to radiate heat to the point of it being said that “when rain was the heaviest, a hand-breadth before his hand and another behind his hand what would be in his hand would be dry, by reason of the greatness of his heat; and when the cold was hardest on his comrades, that would be to them kindling to light a fire.”
Kay growing power isn’t just to “giant size”, specifically he is said to be able to grow “as tall as the tallest tree if he pleased”. By the way the tallest tree recorded for us modern folk is 379.5 feet ( 115.7 m ) and it’s name is Hyperion.
Now, not named in that post is that it’s said when he “carries a burden, be it great or small, it will never be seen, neither from in front nor from behind”. As well as “none will endure fire nor water as well as he”, though no further explanation is given. Also his sword, while unnamed as far as I know, is enchanted so that any wound caused “can not be healed by any physician.”
Some people claim that the passage “cold will his heart be ever, and there will be no warmth in his hands” indicates some sort of power, but if you look at Irish mythology a “cold heart and cold hands” are used as personality descriptors to indicate a hero’s ability to make and act upon calm or “cold” decisions.