But out of all the goddamned sephira and angels, Why [Keter] and [Metatron] respectively?
My guess is that the sephira is the opposition of Tohka, whose position is of [Malkhut].
Nailed it.
In Kabbalah (where the series takes the Spirits' themes), Keter and Malkhut stand on opposite ends of the Tree of Life (Keter at top, Malkhut at bottom), which fits perfectly with the two girls with the greater chances to win Itsuka's heart.
And speaking of Kabbalah, there are ten sephiroth (and by implication ten Spirits), and now that we have Tohka/Malkhut, Origami/Kether, Kotori/Geburah, Yoshino/Chesed, Kurumi/Binah, the Yamai twins/Tiferet, Miku/Yesod and Natsumi/Netzach, only Chokmah and Hod remain without Spirit equivalents.
Also i just remembered and realized that her surname "Tobiichi" [鳶一] has a number kanji on it- specifically the number 1.
I guess the reference/hint was out a very long time but we only just noticed it.
when I realized that each spirit has numerical kanji thats when I started to speculate about her being number 1 and it was during Miku's arc so not that far off for me and she is... it'll be sad though if she looses her spirit power end of her arc due to timeloops :< if still not mistaken number 2 and number 6 are still missing so question now who from those numbers is number 3 (Kurumi) seeking for... all we know she's looking for number 2 to find number 6?! Number 2 is inside DEM thats for certain
and pretty much all of Kether points to Origami, she knows all about Shidou who represents the "Man" in God's domain and Kether being close to God or God itself is omnipotent, second Kether's representative color is White guess who has it none other than Origami (white hair), welp sadly at the end of the volume she goes inverse and well... it may end horribly so its up to Keima... I mean Shidou to fix the past to pave the present for the future GO KEIMA... I MEAN SHIDOU your goddesses await your return