Aang is trying to discover the identities of the Moon and Ocean Spirit, La and Tui. They were the first - or one of the first spirits - to enter the physical world and remain in it. >Roku states that he knows of no spirit old enough to remember La and Tui, aside from Koh.
Raava and Vaatu must be older or as old as La and Tui, because Vaatu was the first spirit to break into the physical world and allow other spirits to do the same.
I mean there's also Wan Shi Tong as a solid chance, but he's in the physical world and Roku doesn't know about the scroll. >Koh states that La and Tui represent balance of duality - push/pull, life/death, good/evil, ying/yang
Raava and Vaatu cop their duality theme, but instead of dancing together they are locked in eternal combat. They are also blatantly more akin to the good/evil, La just drowns people occasionally. >Roku states that the Avatar State is a defense mechanism that utilizes all the past lives, not Raava
You'd think he'd use that opportunity to tell Aang about her. Even after Korra destroys the past lives, the Avatar State is fine. >Guru Pathik tells Aang that he needs to unlock all 7 chakras to truly master the Avatar State (i.e quick activation at will, not panic hulk mode)
Korra has mastered her control of the Avatar State without this, though it's possible letting go of Mako was just a really easy decision. >In the S3 finale, Roku states Aang has the access to the knowledge of all the previous Avatars, and that he does not know where Aang is
Avatar Wan and Raava should know about Lion Turtles, and their ability to give and take away bending. Aang goes through 4 past Avatars and they don't speak up, so apparently none of them have ever talked to Wan or Raava nor do they feel the need to reach out to Aang at this time.
In another dick move, they also don't tell Aang that there is a lion turtle can give air bending to people.