"My Niigaki-san fan level is one rank (higher) than Miyamoto Karin-chan's. Because we're both members of Morning Musume./Because we are/were members of the same group."
So really, Eripon is only a higher level fan due to situations.
And that made Eripon able to see Gaki alot and bond with her, plus that 2012 Bus Tour event that Gaki had, in which Eripon came and freaked out xD. And she dedicated her past two birthday events (2013, 2013) to Gaki. Not to mention all the merch she has (I'm sure Grape has some too, though.).