To identify the householder, you need to use the REL variable. A REL value of 1 or 2 is the householder.
Note also that the flat version of the data file organizes the data so that the householder is always the "first" person, the person whose characteristics would be identified with a 1, such as AGE1, SEX1, REL1, etc.
In addition, all of the householder's person-record data are included in the HOUSEHOLD file, in variables with the prefix "HH" (such as HHAGE, HHSEX, etc.). Thus, if all you want to do is to filter or tabulate your data based on the householder's characteristics, you can do that using HOUSEHOLD alone, without having to link to PERSON or use the flat file.