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According to this post, the に in 一緒に makes this an adverb. Which means that 一緒 is an adjective. But when can 一緒 ever be (or be used as) an adjective? 一緒人? (conjoined twins, a couple, best friends connected at the hip????)
Note: I have no idea if 一緒人 is an actual word or even grammatically correct. It was just something random I threw out there.
As an adjective, I have only seen 「一緒」 paired with 「の」. It is actually used fairly often.
Example:
一緒の部屋に泊まる (to stay the night in the same room)
一緒 becomes an adjective with の
As far as I know, 「一緒人」 in incorrect. Although there are scarce instances of it found online it makes me think it is neither widely accepted nor correct.
If you want to use 「一緒」 in the context of people, it usually looks something like:
僕らは一緒だ (We're together)
I believe the word 「一緒」 recognizes "two separate entities that are together". There is a word 「同一人物」which is used when specifying someone who has two titles is actually the same person. Notice the different kanji to denote this.
Example:
それらの犯罪は同一人物によって犯された。(The crime was committed by the same man)
