I believe Robin was close to her before she even started being friendly with Star but yeah she and Star seem to talk more your comment explained it well kataralover

How the heck does Robin know her better than anyone? I've never seen anything that would have that be possible at all. She was inside his mind once but he doesn't know a thing about her. Beast Boy and Cyborg were the first ones inside her mind so they already know more about her than he does, although they don't know much. Starfire has switched bodies with her, used her powers, understood why she always conceals her feelings, and Raven even told her about her life and where she's from. So why isn't she the main friend during Raven's season? Logically she should be the obvious choice because she's literally the only Titan that knows anything about her. I think it was just because Robin is the leader and they have to make that clear my making him the main everything. Give it a rest already and give someone else credit when they deserve it.

^That.

I think Robin does know Raven the best. They were both made out to be evil; Robin was supposed to be Slade's apprentice and Raven was supposed to be the portal. But they both turned against destiny and decided to do good. So, in a weird, secretive way, they understand and relate to each other.

Just because Robin was blackmailed to be evil doesn't mean he was made out to be evil. He used to be Batman's sidekick, one of the most iconic good guys ever. Not exactly someone who's supposed to be made out to be evil. Besides, I don't know why Slade wanted Robin since he's actually the least powerful of the Titans. Robin knows nothing about her, Starfire is the only Titan who actually knows her very well.

i agree with KataraLover

Starfire and Raven have even experienced each other’s powers! And Robin has no connection or understanding of Raven’s life while Star has experienced some of it.

Yes, and Beast Boy and Cyborg went inside her mind once. If anything, Robin knows her the least.

The girls obviously know each other a lot!