Figment of his imagination
Why are people calling Eve this? She is objectively real. She exists, and cannot not exist if the events of the manga are to make any sense. Aside from Ibusuki's independent confirmation of her existence, she frequently obtains information for Tarou that he could not possibly have obtained on his own. And, oh yeah, she has literally
physically interacted with people. If you try to claim she isn't just as real as any other character, you're the one with an overactive imagination, not Tarou.
For Tarou to reject Ebino is the GOOD outcome, because to accept her would doom Eve. I will readily say that if Tarou, knowing what Eve's fate would be, were to accept Ebino, it would be a deeply immoral act.