Within MM I'd say that Reina might have clung to Koharu in desperation. But, on Koharu's graduation she did tell about how much she liked how Koharu was so individualistic etc.
I don't know when she got close to Aika though, but Reina (and Aika) have both said that they were very close. Reina herself said to Sayu at some point that before when they didn't talk - they didn't really have to, as Sayu had Eri and Reina had Aika. I don't remember Aika's graduation very well but I think she said something along the lines of "Who should I talk to now?" though.
She was also very close with Junjun, in a sister-ish relationship. She was able to relax with Junjun and apparently asked to sit close to her on trains et cetera.
When it comes to outside of MM, she's close with Mai and Miyabi, I think? Recently also Chisato. For a while there was talks about her getting along really well with Maasa too. From backstage footage is seems she's on quite good terms with Nakky too but that might just have been her having a good day and being social.