A kaffir is somone who disbelives even when they know the truth.
Those who disbelieved among the People of the Scripture and the
polytheists were not to be parted [from misbelief] until there came to
them clear evidence -
The aya above says that somone will not be a kaffir unless clear truth is provided, once somone knows the truth and decides to disbelive, they are a kaffir.
And when a messenger from Allah came to them confirming that which was
with them, a party of those who had been given the Scripture threw the
Scripture of Allah behind their backs as if they did not know 2:101
we can infer from the aya above that a kaffir can also be somone from the people of the book weather it be a muslim christian or jew, if they disbelive when there is proof, they are a kaffir.
“We never punish until We send a messenger” (17:15).”
This ayah in the quran indicates that if one did not recive daawa and they were not sent a messanger, they will not be a kaffir. Since they had no knowledge they were not rejecting the truth.
in conclusion a kaffir can be anyone who rejects truth, even if they are from the people of the book, it would not be correct to call somone a kaffir if they have no knowledge of islam. This would be equivalent to calling the prophets parents kaffireen, which we know is not true. If somone never knew about islam, they are not rejecting truth, they simply have no knowledge of islam, and they will not be punished.