Technically, a second PC in a multiplayer game, which is specifically not an NPC. The intention was to keep it from triggering for any NPC (original or mod), but since this required some quite tricky scripting due to the limitations of the original Infinity engine, it may not be completely foolproof in the case of mod NPCs if they don't follow some of the technical conventions of the original NPCs.
Just to clear the issue and avoid any misunderstanding: no, Roxanne most definitely does not have our permission to host, much less alter or expand the mod (in fact the readme of the mod explicitly prohibits this, as Cam pointed out), and what little I have seen of the added content, it most definitely does *not* represent Meira's or my original vision of Amber or our original style. We are currently working on getting the unsanctioned versions taken down, since there is no justification for them, considering that the version hosted by G3 (with our permission and cooperation) should be compatible with the new versions of BG2.