It's very important that you understand the state of religion in Japan to begin with.
It's true that many Japanese are legitimate Buddhists. However, most Japanese, especially younger ones, do not follow any specific religion. Many do go to Buddhist temples, but it is more for Japanese tradition than for a religious reason. Japan is very different, where the lines between religion and culture is very blurred, and many people do things that are considered religious without knowing the religious reasoning behind them.

It's possible that she is Christian, but it is very unlikely, because Christianity is such a tiny minority in Japan. Going to a Catholic school also doesn't mean much, as many non-Catholics go to certain Catholic schools because they are considered to be very high-class.

Knowing this, as well as Hikki's extremely laid back personality, it is likely that she is not very religious at all, but is very open and receptive to multiple people's different beliefs.