I believe that Galacta Knight is one of the most unknown characters in terms of back-story.
He isn't gone forever at the end of the each of the two fights you accomplish, either.
In Meta Knightmare Ultra (Kirby Super Star Ultra),
"He flies unstably in space, then disappears unarmed in a bright flash.". So he's defeated, but not flat out killed.
In The True Arena (Kirby's Return To Dreamland),
"When Galacta Knight is defeated, he flickers white and retreats.". Again, defeated, but not completely gone.
He also is said to be locked away
forever according to Nova in Kirby Super Star Ultra due to him being deemed too powerful, so it raises the question...
Then why is Kirby and/or Meta Knight not locked away for eternity? This may be because they (Kirby & Meta Knight) yet again only defeated him, and not of gotten rid of him forever. (Resulting in Galacta Knight still retaining his title as most powerful).
Also,
"In the pre-fight screen on the True arena (RTDL), it shows Kirby puffing himself up next to Galacta Knight with a confused look on his face. Galacta is holding his lance up in a heroic pose, standing in a white spotlight. If you'd recall, Kirby is in some sort of pain in the other fight screens. However, Kirby is not being hurt in Galacta's screen."
This may or may not apply to anything, but really everything helps in determining what we could find out.
Interestingly, Galacta Knight is confirmed he is from another dimension and the most powerful warrior in the galaxy, according to the descriptions between the explanation of his character, and the boss fights you encounter.
So this stirs up some more questions such as... Does Magolor supposedly know about Galacta Knight?
There is also something important to note, according to the trivia section on the wiki page for Galacta Knight:
"Dark Meta Knight's pause description in Kirby: Triple Deluxe describes Dark Meta Knight as the "shadow of the strongest warrior", hinting that the title has been taken from Galacta Knight. However, it should be noted that Galacta Knight has been referred to as the greatest and the most powerful, not the strongest."
In addition, the similarity between Galacta and Meta Knight is that, they both summon the same armies, or knights, which is strange.
This leads to many loopholes determining the actual power, strength, and overall character of Galacta Knight
From what info we have, Galacta Knight is basically a Time Paradox and indeed has some kind of relation between Kirby & Meta Knight. Being from the same species to having the same title as Star Warriors. It's basically up to HAL to decide where it goes from here on out...
I also used
http://kirby.wikia.com/wiki/Galacta_Knight for info and more!
Here's extra reference if you'd like :