Chief also made appearance on Nintendo platform via Fall Guys and Brawhalla as well.
That too. Overall, he's on a Nintendo system. No ifs, ands, or buts about it. If Nintendo had an issue with it, they wouldn't allow him to show up. They don't have an issue with these crossover appearances because it's not a legitimate problem.
On the reverse side, Nintendo will deny their characters making appearances on other systems. Sony is usually the same way(they're slightly more open, but they were also more ridiculous, like not allowing Minecraft to connect to players outside of the Sony systems). They both are a different flavor of crazy in that regard. That said, keeping console exclusives is completely understandable.
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Unrelated, but I'll note again that Echoes are not traditional Clones, but specifically unique positions where they (so far) have not just the same weight, but are intentionally glorified costumes for ease of use(they must have the same core bodyshape. The difference needs to be hyper small). Wolf doesn't even play like Fox with barely a few shared animations(and that was during Brawl too. He further changed it up). Echoes have mostly the same animations, and in the same move slots too. Recycled animations being thrown in other areas do not count, nor do them existing count for the label. Dr. Mario is the only one of the actual character clones who could be theoretically an Echo(he fits every bit, though he does have some notably different attributes otherwise). Ken has less changes than Dr. Mario overall(though it's worth noting the change isn't that overall much, but the Doc is less of a clone. Ironically, perhaps). But yes, both of them are a good example of "you could change the label for either of these two and it'd work". This does not apply across the board at all.
Pichu is a different shape. Isabelle is a different shape altogether and cannot be one period. Wolf plays completely differently(he used be a palette swap of Fox... in Star Fox 64. The GameCube era made him very very different). Falco has too many different overall stats for that to work(I haven't compared the models to be fair). Young Link and Toon Link are completely different from each other and also completely different from Link. Roy does not play at all like Marth either, btw. Jigglypuff used to play akin to Kirby with still a fair amount of differences, but now they only share a minor amount of stuff. Captain Falcon was partially based upon Samus, but it's unclear if he took some from the Dragon King beta character too(or how much to begin with). Luigi is nothing like Mario anymore(and keep in mind Dr. Mario also retooled stuff from Luigi since Mario no longer has a Spin). Lucas is vastly different from Ness beyond some similar concepts. ...Never mind Ness, who was a clone of Mario in the first game, plays nothing alike. Ganondorf plays widely different from Falcon. This isn't even taking any other Echo into account(as Ryu is the sole Echo who seems to only have the label due to being the "original gameplay clone", though as noted, he does have less differences from Ryu that Dr. Mario has from Mario, so it's not completely uncalled for. One is legitimately more unique. Still an arbitrary call overall, imo).