The Nine are, as Xur puts it, "unknowable" currently. From what we have gathered they are a collection of entities from a different plane or realm opposite our own. The Witness, as we learned earlier this year, is [spoiler]an entire civilization that used the Darkness they learned from the Veil to become a single entity in order to achieve their goal of perfecting the universe.[/spoiler] The Witness' plans are in opposition to The Nine's existence, and which is why we have received assistance previously. However only 5 out of the 9 actually want to help us, one of which we met already is Starhorse that is in Dares of Eternity. We learned the rest though the Emissary and Drifter's cutscenes and events, which was really interesting but otherwise left us with not much more information. We haven't seen the Emissary since the Prophecy Dungeon. Hopefully they continue that story after the Light and Dark saga is finished in June.