Walter Jr. is the oldest child of Walter and Madge Nelson, and the brother of siblings Tina and Binky.  Of all the members of the Nelson family, he is probably the least "normal" in appearance, looking like a hulking brute perhaps more befitting the Addams Family or the Munsters than that of a traditional, late 1960s middle class nuclear family.  Walter may very well be a bit big for his age too (which has not been established). 

Walter is an outgoing and friendly kid, if perhaps something of a dumb and bumbling fool (and he snores and drools when he sleeps).  He is probably the stereotypical "bull in a china shop" on the Nelson family crime capers as well.  He tends to not be cognizant of his size and strength (typical of a dumb brute), and seems to favor black clothing (a black t-shirt and black overalls appear to be his preference).  He wears his hair in a straight bowl cut.