I remember there's a hero-worshipping girl named Rebecca who wants to join your party but is forbidden by her mother. Can anyone remember if this is in SoA or ToB? I'm thinking ToB but I can't remember for sure.

Pretty sure she doesn't appear in the vanilla game. Is the encounter possibly added by UB or something like that?

According to `weidu --strfind rebecca', TLK references to her are at around #30000, so into SoA territory. Some NI hackery should tell more.

Btw I based my statement that she did not appear in the vanilla game on a post by CamDawg here (second one down).

Possible, but he's talking about somebody named Becky and Diane there. Their strrefs are almost in the same section of the tlk, so they might be well related.

<...> Yes, it's related: Becky.cre has name Becky but calls herself Rebecca in her dialogue and wants to join you, whereas Diane.cre is the mother and doesn't want her to leave. The Mithrest waitress (wwench.cre) has name Rebecca but is unrelated. As Cam said, they aren't in the plain game.

Bonus points if you can name who Stivan the Hunter is and where is he found in the game

Is he the halfling Seagull slayer? No idea where he appears though.

Bridge district (x4845, y2049) during the day.