Yup, I really DO NOT like senpai. Kouhai-chan can be a partypooper but least she's an adorable partypooper, Intern-chan is Intern-chan who is wingwoman/slight-indication or rival, but Senpai? She's basically the FPS Smurf (had her naked body nearly seen, and now a kiss, basically the typical archetype "me and Protag-kun got history with each other so I'm superior than you") whose only purpose is to be an eyesore.
My take on Sempai is that she's already missed her chance with Douki-kun - and as far as I can tell, it was for coming on too strong, an approach that he doesn't seem to respond well to. Her actual role in the story at this point is to make Douki-chan freak out with her tales of her past history with Douki-kun, not to be an actual threat to her in the present day.
