[FairyKing]

Wow, even the positions of the swords on the Tree of Life seem to match either aspects of the sword itself or the rider it belongs to:

Jikokuken Kaiji/Malkuth - all of space and time

Ankokuken Kurayami/Yesod - dreams, like the visions Kurayami gives its wielder

Suiseiken Nagare/Hod - logic and rationalization, fitting how Rintaro is among the more intellectual of the riders

Fusouken Hayate/Netzach - aspirations, like Ren's constant drive to grow stronger

Raimeiken Ikazuchi/Teferet - the beauty of reality, which Kento lost sight of at the same time he switched to a different sword

Kaenken Rekka/Geburah - might (like Almighty!) and justice, both very fitting associations for the main rider, and it's also associated with fire and the color red!

Onjuuken Suzune/Chesed - this one's a bit of a stretch, but love and giving could symbolize Daishinji's love of his craft and the support he gives the other riders as a swordsmith

Mumeiken Kyomu/Da'at - a blank space, the perfect place for the unmarked sword of nothingness

Dogouken Gekido/Binah - understanding and development, like Ogami's experience as a veteran swordsman

Eneiken Noroshi/Chokhmah - formlessness, like smoke, and also like Reika's changing role from manipulator to overt enemy to ally

Kogouken Saikou/Kether - infinity incomprehensible to mortals, suitable for Yuri being ancient, no longer purely human, and strange compared to the modern characters.

But then for Xross Saber, I think the "crossguard" explanation is probably better, since the sword's most notable feature is its crossguard being a moving part. I could also see it being about "crossing swords" or a "crossover" since it's created by all the other seiken. Featuring Saber goes especially well with the crossover theme. If it was based on the Christian cross, that would be... yeah, a really weird choice.