I have just realized that there are several BGi characters making appearances in SoA. Tiax, Quayle, Garrick, Brennan Riesling, Faldorn, Ajantis (he is one of the *** you*** in Windspear Hills at the beginning). I am trying to remember if there are any more...
The following NPCs from BG1 can join your party:
Imoen
Minsc
Jaheira
Viconia
Edwin
Sarevok (TOB)
The following NPCs from BG1 make an appearance in BG2:
Garrick (outside the Order of the Radiant Heart)
Faldorn (Druid Grove)
Coran (Forest of Tethyr)
Safana (ditto)
Xzar (Docks District)
Tiax (Spellhold)
Quayle (Circus Tent)
The following NPCs from BG1 appear in BG2 (or they are prominently mentioned), but they're dead:
Dynaheir (Minsc mentions her several times)
Khalid (you can see his body)
Ajantis (Keldorn identifies his body after you kill him)
Montaron (you can find his body in a box inside the Harper compound)
The following NPCs from BG1 do not appear in BG2 (although some of them are in the tutorial):
Branwen
Xan
Yeslick
Eldoth
Skie
Kagain
Shar-Teel
Alora
Kivan
You can meet a few non-joinable NPCs from BG1 in BG2:
Tazok (Windspear Hills)
Calahan (Brynnlaw)
Brennan Risling (if he is indeed the same character)
Dradeel (Spellhold)
Shank & Carbos (Bridge District)
Neb (Bridge District)
Drizzt (Chapter Six)
Elminster (Harper subplot/TOB)
Volo (TOB)
Gorion (TOB)
Biff the Understudy (Playhouse)
The following NPCs from BG1 have the same names as NPCs you meet in BG2:
Tarnor the Hatchetman
Gorf
Carston
Hareishan
Arkanis Gath
Perhaps there should be a spoiler warning attached to this thread?
There are also mods that make several Bg1 NPC's playable in Bg2
Over at Pocket Plane there is Kulyok's Xan Romance
At G3 Domi's Kivan and Deheriana
At CoM Blood Raven is working on updating her Alora mod, and Magnus is developing Yeslick.
I believe there was work on a Shar-Teel project but that has been abandonned, although I think that there are German mods that add both Shar-Teel and Ajantis.
at the beginning). I am trying to remember if there are any more...