senior vs senpai

senior and senpai both are nouns.

Word	Noun	Adjective	Verb	Adverb
senior	Yes	Yes	No	No
senpai	Yes	No	No	No

As nouns, senpai is a hyponym of senior; that is, senpai is a word with a more specific, narrower meaning than senior:

Other hyponyms of senior include dean, doyen, doyenne.

senior (noun)	senpai (noun)
an undergraduate student during the year preceding graduation	someone who is older or other superior to the speaker
a person who is older than you are

senior (adjective)	senpai (adjective)
older; higher in rank; longer in length of tenure or service
used of the fourth and final year in United States high school or college
advanced in years; (`aged' is pronounced as two syllables)