These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Foe Yay: Tama has done this twice with Yuuto. Once during the ski resort arc (and in her grown-up form. Naked. In the hot springs.) and another after he saves Rinko and the Class President (Shimamura) from their deaths at the ferris wheel.
In the anime, Tama instead leaves bite marks much like a vampire's...in Himari's chest, the first time they meet. The second time, it's in the neck. Toned down for TV, obviously.
For some, Shizuku's very, very long and snake-like tongue probably qualifies for this, too, if it isn't Fetish Fuel. Though they both fuels may overlap...
OT 3 - Yuuto/Himari/Rinko, Yuuto/Himari/Kuesu, Yuuto/Liz/Kuesu, Yuuto/Rinko/Kuesu, the list goes on and on.
Likely an OT4, maybe even OT5. Yuuto/Himari/Shizuku/Kuesu, and maybe Rinko or Lizlet added in. Tama, if he can convert her.
The most likely grouping would be a OT4 of Yuuto, Rinko, Himari, and Kuesu. Rinko because he knows her so well and has known her for so long; Himari because she was the first to know and befriend him (and she's the manga's title); and Kuesu because she was the first human girl that he met (who also stole his first kiss).
It's an Improbably Female Cast. There are two kinds of guys, the obviously male, and the kind who are easily confused as female. There's even an Unsettling Gender Reveal, since the characters are as mistaken about the gender of that boy-youkai as the viewers are.
Sasa, the one-legged youkai in human form? The one with the frilly clothes and skirt? It's a guy.
What Measure Is a Non-Badass?: Yuuto is often disliked for letting the girls do all the fighting and always needing to be saved. While he does have powers, they seemingly only show up when he or someone important to him is in mortal danger. THEN, however, any weapon he wields gets magically enchanted, and he is able to summon a Pillar of Light, knocking out even a raging Himari. Also, as of Chapter 43, he's coordinating their attacks against Tama.