Now I have seen people call だ the only copula, even excluding its adverbial or attributive forms で and な (and seemingly の), with the argument that only predicative copulas, which are necessary for predicative expressions, can be called "copula".
Others include them in the term "copula" and simply differentiate between "Conclusive" and "Adnominal".
However I have not found a discussion on this site about the suffix that follows the stem of adjectives in their 終止形, well, predicative form. It seems to me, that this suffix, which can be inflected, works like a copula.
Some claim that in ...が綺麗(です) and ...が可愛い, there is a predication in both, but in the second case, い is responsible, not any ommitted です, for which to use we would have to nominalise the expression anyways: ...が可愛いのです/だ, although です has sort of been accepted to use behind an adjective despite being technically incorrect. In short: ～い = "to be"?
However, some things make me question this: A sentence doesn't need a copula(call it a verb or not) OR verb to be grammatically correct, meaning, an adjective in its 終止形 form is a whole sentence. But we cannot ommit the suffix.
But then again, attributively, we inflect い and don't add any だ-variant. This argument depends on whether you think attributive versions of だ are even "true" copula.
So, I am conflicted. Could you call い some kind of copula? Maybe a suffix with copulative properties?
Any linking to a question which already answered this would be helpful, too.