Well, Reina isn't really known for being close to anyone.
And honestly, from the younger (6ki and down) kids neither was Miki. But apparently, I think she said this in a radio? I don't remember, Miki did give Reina advice and helped her out backstage when they just joined since Sayumi and Eri found each other immediately and Reina was "left out" and Miki has experience etc.