@Knighto He's done that only with people that kinda knew about her already.
Hiki guessed that his work is based on a real story and saw her once, Irifune was the same and she is an authority on ghosts and Hana not only met her, but also lived with Bambi for a while, so hiding Ako wold be a chore.
With normal people like Gakuto it would be a pain explaining the whole ghost thing, especially without letting them meet her or explaining the circumstances of Yamashiro. And, it might be just my feeling, but I'm not sure if Bambi would want to introduce his ghost
girlfriend to his friends for a number of reasons.