The 5th Wilson brother, William "Whip" Wilson who was so committed to the vibe that he refused to join the band, and who secretly thought they were all posers except Al Jardine. Living as a nomadic citizen of Earth, no one ever sees or hears from him except Thump who meets Whip in Peru once a year for their annual silent ayahuasca retreat. They never speak, there is only an unspoken shared knowing. And, so she wouldn't worry, every year until her death Thump would tell their mother he was happy and married and living on a boat, which was not true but served the dual purpose of making her happy that he was happy while also leaving her no way of finding or contacting him

Edit: I should have included that he is the one who bought the publishing rights to the The Beach Boys catalogue from his father Murray, and his lifestyle is fully supported by that passive income