On the Japanese Wikipedia, the topic of Halo has its own logo.
If Master Chief were to get in, it would be because of a Halo game on Switch, although that is not necessarily guaranteed. Nintendo tried to push for Rayman Legends to do well in Japan, and he still is not in Smash.
A character is probably chosen on their fighting potential and how well Japanese audiences will receive them. Because Rayman is mostly purple and that colour is associated with royalty, that is probably what is stopping him from having appeal to Japanese audiences.
As far as Master Chief goes, I don't think his character design will prevent him from being well received by the Japanese. They can even make him into a sword wielding anime character if they wanted.