means a person, other than a mortgagee not in possession, who, whether in his own right or as a trustee for any other person and whether alone or jointly or in common with any other person, is entitled to receive the rack rent of the dwelling, or where the dwelling is not so let, would be entitled to receive it if it were so let; and

is not himself liable as lessee of the dwelling, or of property which includes the dwelling, to pay such a rent to a superior landlord.