Just a random thought but wouldn’t it be easier to write for Hii instead of Fu since he would definitely be the dominant mean type?
So let me give a bit of a nuance on my thinking of Fu and Hii.
I don't think Hii is Fu's vital instrument per se. Yes Hii is Fu's treasured object and all but the voice he seems to have? I personally think think that's all in Fu's head.
Hii and Fu are a Jekyll and Hyde kind of situation or think of Jim Carrey in The Mask. When the mask is off, Carrey's character isn't the most confident or smooth with the ladies but with the mask is on, he's a super confident womanizer.
Now I don't think Fu has a split personality, more like the voice inside his head gives him the confidence he needs in times when he's solo. If you have read the manga during the latest arc, Fu became Hii due to Amo's need in him and not Fu's need in Hii.
Fu has said he doesn't want to make decisions on his own in case he makes the wrong choice and the repercussions are bigger than the initial outcome. That's where Hii comes. Think of it as liquid courage without the alcohol.
All in all, Hii doesn't truly show up unless in dire situations or asked upon like when escaping the flying trash beast, helping the cleaners in the broker arc of the manga or assisting Amo in protecting Rudo in the last arc of the manga.
When having sex with Bunny? Fu's more likely to shut down than to think of how to improve the situation. Hii isn't needed to escape because Fu's not in any actual danger and he hasn't been called upon either. He's just a nervous virgin mess with a hot girl who's willing to let him lose his V card without her knowing.
Also a small side note in all this, I think if Fu did use Hii on Bunny, as someone who's not seen as super trustworthy in the Cleaners eyes just yet. Enjin would def beat the shit out of Fu if he tried to alpha dog her lol
I hope I made sense in why Hii wasn't in the scene. Sorry for the huge ramble and also the manga spoilers.