Here's a list of all the characters:
Joinable non player characters
* Dogmeat
* Jericho
* Star Paladin Cross
* Fawkes
* Charon
* Clover
* Sergeant RL-3
* Butch DeLoria
Agatha's House
* Agatha
Alien Crash Site
* Dead Recon Craft Theta Alien
Andale
* Old Man Harris
* Bill Wilson
* Martha Wilson
* Jenny Wilson
* Jack Smith
* Linda Smith
* Junior Smith
AntAgonizer's Lair
* AntAgonizer
Arefu
* Evan King
* Brailee Ewers
* Ken Ewers
* Karen Schenzy
* Ian West
Bigtown
* Red
* Shorty
* Dusty
* Flash
* Bittercup
* Pappy
* Timebomb
* Kimba
* Sticky
Brotherhood of Steel members throughout Fallout 3
* Paladin Hoss
* Initiate Pek
Canterbury Commons
* Uncle Roe
* Joe Porter
* Dominic D'Ellsadro
* Machete
* Derek Pacion
Caravan Merchants
* Crow
* Doc Hoff
* Crazy Wolfgang
* Lucky Harith
The Citadel
* Elder Owyn Lyons
* Scribe Rothchild
* Star Paladin Cross
* Paladin Gunny
* Paladin Bael
* Knight Captain Durga
* Knight Artemis
* Scribe Peabody
* Scribe Bowditch
* Scribe Jameson
* Scribe Vallincourt (Broken Steel)
* Sawbones
* Squire Arthur Maxson
Dukov's Place
* Dukov
* Cherry
* Fantasia
Galaxy News Radio
* Three Dog
* Margaret
* Knight Captain Colvin
* Knight Dillion
* Knight Finley
Girdershade
* Ronald Laren
* Sierra Petrovita
Lyons' Pride
* Sentinel Sarah Lyons
* Knight Sergeant Wilks
* Knight Captain Dusk
* Knight Captain Gallows
* Paladin Glade
* Paladin Kodiak
* Knight Finley
* Knight Dillon
* Paladin Vargas
* Knight Captain Colvin
* Initiate Reddin
* Initiate Jennings
Megaton
* Andy Stahl
* Colin Moriarty
* Confessor Cromwell
* Deputy Weld
* Deputy Steel
* Gob
* Jenny Stahl
* Leo Stahl
* Lucas Simms
* Billy Creel
* Maggie
* Megaton Settler
* Mister Burke
* Moira Brown
* Mother Maya
* Nova
* Harden Simms
* Stockholm
* Wadsworth
* Walter
* Lucy West
* Rory McLaren
* Nathan Vargas
* Manya Vargas
Meresti Metro Station
The Family
* Alan
* Brianna
* Holly
* Ian West
* Justin
* Karl
* Robert
* Vance
Paradise Falls
* Eulogy Jones
* Carolina Red
* Cutter
* Forty
* Grouse
* Jotun
* Pronto
* Ymir
* Clover
* Crimson
* Bronson
* Miss Jeanette
* Bleak
* Breadbox
* Rory McLaren
* Carter
* Frank
Raven Rock
* President John Henry Eden
* Colonel Autumn
* Anna Holt
* Nathan Vargas
Republic of Dave
* Dave
* Bob
* Rosie
* Jessica
* Rachael
* Shawna
Rivet City
* Abraham Washington
* Angela Staley
* Anna Holt
* Armitage
* Bannon
* Belle Bonny
* Brock
* Butch DeLoria
* C.J. Young
* Carlos
* Cherry
* Christie Young
* Cindy Cantelli
* Diego
* Dr. Zimmer
* Father Clifford
* Flak
* Gary Staley
* Garza
* Harkness
* Henry Young
* Horace Pinkerton
* James Hargrave
* Janice Kaplinski
* Lana Danvers
* Madison Li
* Mei Wong
* Mister Buckingham
* Mister Lopez
* Paulie Cantelli
* Preston
* Private Jones
* Seagrave Holmes
* Shrapnel
* Sister
* Tammy Hargrave
* Ted Strayer
* Trinnie
* Vera Weatherly
* Victoria Watts
Springvale
* Silver
Apostles of the Holy Light
The following character appear only with the Broken Steel add-on
* Atom's Champion
* Brother Gerard
* Mother Curie III
* Novice
* Sun of Atom
Temple of the Union
* Alejandra Torres
* Bill Seward
* Caleb Smith
* Four Score
* Hannibal Hamlin
* Simone Cameron
Tenpenny Tower
* Allistair Tenpenny
* Bessie Lynn
* Doctor Banfield
* Mister Burke
* Herbert Dashwood
* Anthony Ling
* Chief Gustavo
* Edgar Wellington II
* Godfrey
* Irving Cheng
* Lydia Montenegro
* Margaret Primrose
* Michael Masters
* Millicent Wellington
* Roy Phillips
* Susan Lancaster
* Shakes
* Tenpenny Residents
* Tiffany Cheng
Tranquility Lane (Vault 112)
* Stanislaus Braun (as Betty)
* Timmy Neusbaum
* Roger Rockwell
* Janet Rockwell
* Martha Simpson
* Mabel Henderson
* Old Lady Dithers
Underworld
* Ahzrukhal
* Barrows
* Carol
* Cerberus
* Charon
* Crowley
* Ethyl
* Greta
* Griffon (only with the Broken Steel add-on)
* Meat
* Moira Brown
* Nurse Graves
* Patchwork
* Quinn
* Reilly
* Snowflake
* Sydney
* Tulip
* Willow
* Winthrop
Vault 101
* James
* Alphonse Almodovar
* Amata Almodovar
* Mister Edwin Brotch
* Officer Herman Gomez
* Freddie Gomez
* Butch DeLoria
* Stanley Armstrong
* Andy
* Beatrice Armstrong
* Jonas Palmer
* Lucy "Old Lady" Palmer
* Allen Mack
* Wally Mack
* Gloria Mack
* Tom Holden
* Mary Holden
* Officer O'Brian
* Officer Richards
* Officer Wilkins
* Ellen DeLoria
* Pepper Gomez
* Paul Hannon Sr.
* Vikki Hannon
* Paul Hannon Jr.
* Officer John Kendall
* Christine Kendall
* Floyd Lewis
* Officer Taylor
* Grandma Taylor
* Officer Wolfe
* Officer Park
Warrington Station
* Bessie Lynn
* Michael Masters
* Roy Phillips
Operation: Anchorage
Anchorage Reclamation Simulation
* General Chase
* Benjamin Montgomery
* Adrienn Adami
* Lieutenant Morgan
* American War Correspondent
* Quartermaster
* General Jingwei
Outcast Outpost
* Defender Sibley
* Protector McGraw
* Defender Morrill
* Specialist Olin
Point Lookout
Capital Wasteland
* Catherine
* Tobar the Ferryman
The Ark & Dove Cathedral
* Croatoa
* Jackson
* Jimson
* Nadine
* Woodrose
Blackhall Manor
* Obadiah Blackhall
Calvert Mansion
* Desmond Lockheart
* Freki and Geri
Coastal Grotto
* Jacob Humboldt
* Rip Smithy
* Plik
Disaster Relief Outpost
* Marcella
Haley's Hardware
* Haley
Kenny's Cave
* Kenny
Marguerite's Shack
* Marguerite
Pilgrim's Landing
* Panada
* Nadine
St. Aubin Medical Facility
* Professor Calvert
Ruzka's Lair
* Ruzka
The Pitt
Capital Wasteland
* Prosper
* Ramsey
* Wernher
Haven
* Ashur
* Marie
* Sandra
The Mill
* Bone
* Brand
* Everett
* Faydra
* Grudd Bear
* Gruber
* Hammer
* John Bear
* Marco
* Spook
Downtown
* Adan
* Bingo
* Friday
* Harris
* Jackson
* Kai
* Mex
* Midea
* Milly
* Nola
* Slave Master
* Trouble Man
Uptown
* Duke
* Krenshaw
* Lulu
* Mona
* O-Dog
* Vikia
* Phantom
* Reddup
* Squill
Steelyard
* Wild Bill