Named for O.J. Simpson's former house guest Brian "Kato" Kaelin, this is a person who seeks out rich and/or famous people, moves into their home rent-free and is given things like gifts, expense accounts, credit cards, free vacations, professional and political connections and other perks without giving anything in return. This is not the same as prostitute or golddigger as sex is not involved as it can be with a person living with someone of the same gender and both are heterosexual.
Also, this person isn't your garden-variety leech as these individuals purposely seek out rich and famous people for their own personal gain, perhaps to become famous themselves.
Kato Kaelin with O.J. Simpson, Kim Kardashian with Paris Hilton.
That dude is such a Kato as he's getting everything handed to him.