Again, sorry if this is already quite well known, I've been avoiding speculation about this game, but every character so seems to be themed around opposite pairs:

Two playable characters, Reimu and Marisa, representing the archetypes of the Fool and the Magician: talent and hard work

Ubama is the boss of the yamanba and has the power to destroy sanctuaries, contrasted with Sakata, who has no interest in society and can create sanctuaries: extroversion and introversion, creation and destruction

Chimi has the same surname as Nue and may well be her mother, she's unidentifiable like her daughter, but that's because she's older than the concept of youkai itself, while Nue is clearly a youkai: parent and child, old and new

Nareko is Japanese, but cosplays as an Egyptian, and she's a dousojin, which are often represented as a human couple: east and west, male and female

And if my silly theory about Sariel and Shinki is right: good and evil, life and death

There seems to be a yin-yang theme to this game, and to Touhou in general, of course, but it's especially prominent here