The characters that you either unlock through choices or people who you will eventualy get:

-- Handmaiden/Diciple (male 4 handmaiden, female for diciple)
-- Hanharr/Mira (dark side for hanharr, light or neutral for mira)
-- Hk47 (build him by finding all the parts
-- Gotos droid ( do nar shadar eventualy you will be captured by goto an when you complete it you get one of his droids)
-- Visas (get yourself at least 25% either light or dark)
-- Boa-Dur (meet him in the restoration zone)
-- Mandalore (meet him on Duxen)
-- Kreia, Atton, T3-M4 (the first characters you meet all on Peragus)