[Twitter] Who is Heechul's national sister?

Heechul's first tweet after SS3 Beijing includes F(x) Sulli. Sulli came to SS3 and did a duet song with Super Junior's Kim HeeChul.

Heechul uploaded a picture of him brushing Sulli's head and tweeted:

National sister Sulli who came to China even though she's busy! She work hard at everything she does and is loyal! Even though she looks like a baby, she's my dongsaeng who has deep heart like my eyes~ She always says "I only receive help from you everyday, I feel bad ㅎㅎ" If you are that thankful give me cash go go