Should we consider "sqlplus" as an Oracle client?
SQL*Plus is a client that is already available for a very long time. If you use sqlplus username/password then it does not use the tnsnames.ora. It connects to the local database that is defined in the ORACLE_SID environment variable. If you use sqlplus username/password@SID then it will search in the tnsnames.ora file to find out the location of the SID database.