Hmm, ... it ain't altogether inconceivable that everyone else at the company party *cough-cough* conspired to achieve a 'Senpai x Kouhai' hookup, so that things proceeded generally like this:
Phase 1 - Watanabe-senpai got drunk.
Phase 2 - Kobayashi got (a-hem) "nominated" to get her safely home.
PROFIT!
In The End, possibly because company policy might frown on those sort of relationships between senior & junior staff, the new couple decided to 'maintain the status quo', until outside working hours (wink-wink, nudge-nudge).
That's a nice title, I especially like the ❤ mark.
Also, using her status as boss to ensure he complies with her instruction to wash her body? Interesting. Alcohol can at times truly be a great wingman.
And hey, considering that ending...I'm inclined to believe that they actually have a really good relationship with each other.
Me: And you will love it, you lucky bastard!