I do agree she is pretty friendly with the idols. In fact, even the other idols respect her. For Example, Chihaya actually calls her "Leon-Sensei" towards the end of her DLC EX episode.
That's exactly the point I started to think she's more of a friend.
It's certainly more of a friendly rivalry. Just because some people are rivals doesn't mean they cannot be friends. Rivalry is just a method of competing against one another, but competition doesn't always include malice.
Btw, while I do agree Leon is probably the nicest rival we've had in the whole series, I think I remember Takane and Miki (outside Azusa's route) weren't exactly that bad in SP. It was Hibiki who was the jerk of the trio.
I've never played SP before, but according to wiki and some other information, they're not anything close to Leon as far as I can tell.