elder vs senpai

elder and senpai both are nouns.

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

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

Other hyponyms of elder include dean, doyen, doyenne.

elder (noun)	senpai (noun)
a person who is older than you are	someone who is older or other superior to the speaker
any of various church officers
any of numerous shrubs or small trees of temperate and subtropical Northern Hemisphere having white flowers and berrylike fruit

elder (adjective)	senpai (adjective)
used of the older of two persons of the same name especially used to distinguish a father from his son