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I love the idea that Tsubaki supposedly arranged to be Kid's weapon when both of them still young. Got this idea when Mosquito states in Ep 28 that Nakatsukasa family is adopted by Shinigami and I imagined both of their family relationship is similar to political alliance.

Other than that, Nakatsukasa family is famous for their multiple weapon form and Arachne herself directly create the family's weapon and because of that, I consider Tsubaki the strongest weapon.

It's pretty cool idea that Kid is supposedly to become Tsubaki's Meister because like the reason above, I love to consider Tsubaki as Shinigami for weapon just like Kid is Shinigami for Meister.

Then, Blackstar came and Shinigami decides that he is better Meister for Tsubaki since both of them can create stronger resonance. Based on the lore, the greater the difference between partner, the stronger resonance they can create.