I did part 2 training that starts the blackmail quest but not further. Would be nice if the game had a quest listing with the others that tell you that like I have seen for others.
Well, in some sense I guess the game opens up a little too early. Bill is clearly meant as the tutorial/setup. It makes no sense to test your powers if you have already enslaved somebody.
At the moment I don't think there is anyone else that has already been introduced, but are still locked behind others quest, without given a hint about it.
Billy opens Scarlett, Chloe, Eleanor and Kobe
Sofia opens Cara and Alice, Cara is clearly connected to Sofia and Alice's quests begins immediately, you just can't proceed.
Eleanor opens Luna, which we have briefly interacted with, but not really. In the future her friend is also available.
Scarlett opens and introduces Mary and the now removed quest with the shower girls, but we can still do the first part.
Kobe possible introduce and opens up for Sharon
A main quest opens up for Linda
That is the lot if I am not mistaken.