Tsukumogami insecurities... or lack thereof. (LunaBen + RaiLyri)
HEADCANON TIME:
Due to the intrinsic nature of their backstories, tsukumogami are often associated with abandonment issues or insecurity. Examples: tryhard Kogasa, vengeful Medicine.
Though she may have been insecure in the past, I see Raiko as a strong example of overcoming this.
Having already "lived for a while" as a humble tsukumogami, Raiko in DDC boldly reinvents herself under the ideal of personal freedom. She feels experienced and self-assured nowadays. Drums are strong, straightforward instruments, fitting to embody a pure confident vibe.
I think the Tsukumos, brought to life suddenly, are less secure.
The biwa is an instrument with deep, harsh distortions. Yatsuhashi comments on this in "Grimoire of Usami," citing her koto's sounds as more gentle… which surprised me, since Benben's personality is supposed to be the gentler one.
Biwa often accompanies dramatic vocal narratives. What come to mind are spooky and violent stories like "Hoichi the Earless" (which is referenced in Benben's spellcards). Maybe this tonal gap with her elegant demeanor is what makes Benben feel suitable for an interpretation that's just a bit "off".
Perhaps Benben is unexpectedly unstable, and Lunasa's aura makes a good match to keep her calm and on-track.
…Or perhaps the implication of the comic is, instead, Benben just has a dark sense of humor, which can challenge and intrigue even someone as dark as Lunasa.
By the way, despite her darkness, Lunasa's profile describes her as honest to a fault, so she would NEVER cheat! "Yandere" vibes are the funniest when they're totally unnecessary.
I wrote a serious nuanced fanfiction ("Heartstrings") about Lunasa & Benben helping each other through insecurities-- but this is a more jokey angle on it, LOL. Either way, it's fun to pursue some twist to the Tsukumos' personalities, which don't often get developed!