If a user is showing under Unmatched Core (Core > Settings > Blackbaud Connections > Connect Education Edge > User Records: Unmatched Core), this indicates the user record in Core has one of the roles that are synced through Connect EE, but they are not linked to an EE record.
This can happen if the user is either entered directly in through Core, was never linked, or if they were previously linked but have since been unlinked through Matched users. If the record exists in EE, we would expect to see it as an unmatch Education Edge record as well where it could be linked.
Once the record is in EE, it will show under unmatched Education Edge users and from there, you can link it to the Core record manually. You can't manually link them if they are just unmatched Core users.