leftover vs remaining

leftover is a noun but remaining is not a noun.

Word	Noun	Adjective	Verb	Adverb
leftover	Yes	Yes	No	No
remaining	No	Yes	No	No

As adjectives, leftover and remaining are synonyms defined as:

Other synonyms of leftover include left, left over, odd, unexpended.

leftover (noun)	remaining (noun)
a small part or portion that remains after the main part no longer exists

leftover (adjective)	remaining (adjective)
not used up	not used up