replayed kirby fighters 2 story mode. i think it's cute how you can see ddd and mk getting closer throughout the chapters through the difference between their defeat animations, title, and the complexity of their combo attacks.
in the first time you face off against them for real, they're called "old rivals" because that's their connection at the start, they're both rivals of kirby, banding together in hopes of taking him down. even prior to kf2 the times they worked together was also to take down kirby (or to help kirby save the universe). the two haven't really been connected to each other. a notable display of their weak connection during this time is that when they're defeated, they retreat separate ways even though their destination is the same place.
the second time you fight them, they're stronger and a lot more in sync. their bond isn't just about taking kirby down now, hence the title change to "sworn partners". the specific word used to describe their bond in japanese is "盟友" (meiyū), which denotes a profoundly deep ride-or-die relationship (makes sense considering the two of them put on very obviously cursed masks during their final fight against you). when they're defeated this time, though they are too weak to retreat this time, much less move, they try reaching for each other anyway
i especially like the credits, where mk and ddd take a walk after their defeat, showing how their bond now extends beyond just fighting kirby