What a lovely word senpai is. A term of respect in Japanese, it is used mainly in the school and the workplace situations to address anyone superior in status--teachers, for example, bosses, or even relatives when they are older than oneself. To be someone's senpai is to out rank them, to be senior to them.
And so, depending on the situation or relationship between people, this term might refer to any of those ways a person could be grander with regard. In this case we could take as an example teacher and student, where the teacher is 'senpai' while their pupil becomes one 'kohai'.