i dont really consume the fanon content for metadede often (it tends not to be my cup of tea/line up with my interpretations for them) but they're relationship is something very fun to explore
they're literally anything but romantic to me tho
that being said in my interpretation their relationship is like:
he's his knight, he's his king, he's just a boss to him, he'd kill for him, they raised a kid together and he ended up trying to take over the planet, one is a laid back king perhaps maybe too laid back, the other is a permanent lone wolf despite LITERALLY HAVING A CREW, they know each other so well but they don't even know each other's favorite colors, they both have beef with the same kid that has and can beat them up into a pulp, they've never been straight up open to the other because god forbid. that is way too intimate, they respect each other now but it wasn't always that way
like. to me, Meta Knight just showed up on Popstar one day looking for a job and resources in order to build the Halberd and managed to end up working for DDD. it was nothing but business, and mk had absolutely negative respect for ddd because he's a shit king, but comes around Kirby's Adventure ddd actually does his duty as king and tries to protect his citizens and for once meta knight sees him more as just his boss or some tyrant, they get closer somewhat, the boss and employee relationship wanes into something more casual and then BOOM Dark Matter Trilogy comes along and sets everything back. Meta Knight grows skeptical of Dream Land's inability to protect itself and attempts a take over only to fail miserably and breaks ddd's trust, which he scrambles to repair by doing "righteous" acts like saving the mirror world (which he fails at) and preventing dark nebula from being released (which he fails at) and it's not only until Return to Dream Land that they properly meet up again, this time on equal grounds and for once they get close enough that calling each other "friend" doesn't sound far fetched