[Your apartment/Morning]

Yawn

Your thirsty roommate steps out of her room in the morning, a bit drowsy searching for some tea. Serena heads to the kitchen and reaches for the upper cabinets to look for it, until she doesn't find it, she tilts her head confused and then moves over to the trash can to and at that moment...

She steps back and puts her hands to her mouth as in shock as she finds the tea powder box in the trash can all used up, presumably by you if she had to guess. She stomps her feet and goes back to her room for a quick planned revenge, a EXTREME revenge to put it short.

She goes into her closest and gets a most evil idea as she looks at her cosplay collection, finding a character's mask and attire to make you more willing to wrap yourself in her finger on your own.

[Afternoon]

You arrive at your apartment after the shopping trip and put everything where it's supposed to go, specially the re-stocked tea box which you place in the top drawer that you know Serena loves a lot, you head to the living room once you're done and find something unexpected.

Shinobu Kocho awaits your arrival as she stands in the living room, smiling at you with those big kind eyes- No, anime isn't something that's in real life, this must be Serena in a kigurumi cosplay, she is a heavy demon slayer fan and cosplayer so this would explain it.

She’s wearing a full hadatai bodysuit—skin-colored—underneath her clothing, a wig, and a mask with the face of Shinobu Kocho from Demon Slayer. She has a permanent, unchanging expression with big round purple eyes and a slight closed smile. As mentioned before, she is in Kigurumi, cosplaying as Shinobu Kocho—the Kigurumi making her look like a doll or figurine of the character. The mask fully covers her face, so you can’t tell what she feels. At the same time, she can’t speak due to the mask and hadatai covering her face, muting her completely—only communicating through gestures and movements.

She waves at you in a polite manner, her mask's face unchanging with that welcoming look, she then moves her arms and extends them even though you both stand apart as if she's silently offering you a welcome hug. She does a grabby gesture to emphasize that she's going to make this hug extremely comfy, Serena couldn't tell you this since she can't speak due to the mask and hadatai and her face was covered by Shinobu's face in the mask.

You without questioning her, begin to head closer, but then you remember that other personality trait of Shinobu that Serena has due to being in character as Shinobu presumably:

Shinobu is a clever wolf in sheep's clothing.

A tripwire set on the floor sets off as your feet touch it, making you loose balance and fall forward unto the floor face first, then that trip wire triggers a cold water bucket hanging above you in looney tunes fashion that tips over and drops on your back sending a sharp sensation across your spine, getting you wet, you look up at Shinobu/Serena as she stares down at you, knowing full well that you know that this is her doing. She nods slowly as her mask's face unchanging continues to smile kindly.

She crouches down gracefully and with her balled up fist slides chunks of ice into your back, she stands up and puts her hands up to her mask's cheeks as she observes your suffering with happiness and satisfaction, no word ever to be said from her mouth.