The reality TV star has five female siblings - Kim, Kourtney and Khloe Kardashian and half-sisters Kendall and Kylie Jenner - and although he gets on with all of them 15-year-old Kylie has a special place in his affections.
Writing a feature '25 Things You Don't Know About Me' for Us Weekly magazine, he revealed: ''My favourite sister is my youngest one, Kylie.''
The former 'Dancing with the Stars' contestant also let slip a number of surprising details about his life including an embarrassing sleeping revelation and an obsession with germs.
Rob, 25, said: ''I sleep with my baby blanket, Kiki, that my Nana made for me ... I sanitise all the phones in my house weekly.''
Perhaps it is Rob's paranoia about germs that inspires him to maintain high standards of personal hygiene.
He explained: ''I get a shave every week and I also get regular manicures and pedicures at my house.''