Kirby is a Kirby, and a Meta Knight is..unknown? Meta Knight and Kirby are similar
in a few ways. I'll make some theories for you.
Game canon theory:
Originally, back in Kirby's Dream Land, (I haven't played this game yet. I'm getting this from Spring Breeze on Kirby Super Star Ultra, so I may be wrong if any of this stuff isn't on Kirby's Adventure.) Kirby started his life on Pop Star. He was having a nice day, until he discovered somebody was stealing all the food in Dream Ladn! Somebody else similar, named Meta Knight. He was unmasked back then. He had no idea about Kirby for a while, and was very hungry, due to having no food. Kirby found King Dedede's castle, and defeated King Dedede. On the other hand, with Meta Knight, he noticed a weird balloon (actually Kirby) above King Dedede's castle. All the food falls out, and Meta Knight wonders about it. King Dedede gets angry and Meta Knight checks him out. Meta Knight asks what's wrong, and somebody named Kirby took all of Dedede's food. He said he looked a lot like Meta Knight.
Kirby's Adventure, Meta Knight has heard rumors and sights about somebody named Kirby. He realized his face was similar to Kirby's, and that Kirby was getting well-known. Meta Knight went to Orange Ocean. He is now angry against Kirby, but thankful he got food for him. He forgets about Kirby for a while, and puts a mask on, so nobody will compare him to Kirby in the bad way. He meets Kirby, so he can battle him. Later, he helps Kirby out, as a thank-you for getting food back. In the later games, he forms more of a rival. Eventually, he doesn't know if he should be friend or foe, so he sticks to both. Meta Knight is still embarrassed, so he keeps his mask on. Later, in Revenge of Meta Knight, he decides foe and tempts to take over Dream Land. He fails, obviously. His mask falls off, and he uses his cape to fly away to somewhere else. He is very embarrassed now, and stays mostly enemies until Kirby's Return to Dream Land. He decides it's too hard to battle Kirby, and maybe he's too harsh. He decides to help Kirby this time, and that's that.
^We all like dots, we all do.