senpai vs elder

senpai and elder both are nouns.

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

As nouns, elder is a hypernym of senpai; that is, elder is a word with a broader meaning than senpai:

Other hypernyms of senpai include senior.

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

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