I’ll now disclose everything I’ve observed about a very extraordinary pantheon of mutants…
—Uatu the Watcher

Featured Characters:

X-Men (First appearance) (Origin revealed) (Only in flashback)
X / Professor X (Dr. Charles Xavier) (First appearance) (Origin revealed) (Only in flashback)
Marvel Girl (Jean Grey) (First appearance) (Origin revealed) (Only in flashback)
Cyclops (Scott Summers) (First appearance) (Origin revealed) (Only in flashback)
Iceman (Bobby Drake) (First appearance) (Origin revealed) (Only in flashback)
Avenging Angel / Angel (Warren Worthington III) (First appearance) (Origin revealed) (Only in flashback)
Beast (Hank McCoy) (First appearance) (Origin revealed) (Only in flashback)
Supporting Characters:

Sharon Xavier (First appearance; dies) (Only in flashback)
Brian Xavier (First appearance; dies) (Only in flashback)
U.S. Army (First appearance) (Only in flashback)
Logan (First appearance) (Only in flashback)
Captain America (Steve Rogers) (First appearance) (Only in flashback)
Magda Lehnsherr (First appearance; dies) (Only in flashback)
Anya Lehnsherr (First appearance; dies) (Only in flashback)
Dr. Moira MacTaggert (First appearance) (Only in flashback)
Gabrielle Haller (First appearance) (Only in flashback)
Katherine Summers (First appearance; dies) (Only in flashback)
Christopher Summers (First appearance) (Only in flashback)
Alex Summers (First appearance) (Only in flashback)
John Grey (First appearance) (Only in flashback)
Elaine Grey (First appearance) (Only in flashback)
William Drake (First appearance) (Only in flashback)
Maddie Drake (First appearance) (Only in flashback)
Antagonists:

Brotherhood of Evil Mutants (First appearance) (Origin revealed) (Only in flashback)
Magneto (Magnus) (First appearance) (Only in flashback)
Scarlet Witch (Wanda Maximoff) (First appearance) (Only in flashback)
Quicksilver (Pietro Maximoff) (First appearance) (Only in flashback)
Toad (Mortimer Toynbee) (First appearance) (Only in flashback)
Namor the Sub-Mariner (First appearance) (Only in flashback)
Axis (First appearance) (Only in flashback)
Nazi Germany (First appearance) (Only in flashback)
Master Man (Wilhelm Lohmer) (First appearance; dies) (Only in flashback)
Numerous unnamed soldiers (Only appearance; death) (Only in flashback)
Cain Marko (First appearance) (Apparent death) (Only in flashback)
Shadow King (Amahl Farouk) (First appearance) (Only in flashback)
Mob of farmhands (First appearance) (Only in flashback)
Elias (First appearance; dies) (Only in flashback)
Numerous other unnamed Ukrainians (Only appearance; death) (Only in flashback)
Korean People’s Army (First appearance) (Only in flashback)
Shi’ar Empire (First appearance) (Only in flashback)
Emperor D’ken (First appearance) (Only in flashback)
Mister Sinister (Nathaniel Essex) (First appearance) (Only in flashback)
Jack O’Diamonds (Jack Winters) (First appearance; dies) (Only in flashback)
Mob of villagers (First appearance) (Only in flashback)
The Right (First appearance) (Only in flashback)
Cameron Hodge (First appearance) (Only in flashback)
Larry Trask (First appearance) (Only in flashback)
Steven Lang (First appearance) (Only in flashback)
Bolivar Trask (First appearance) (Only in flashback)
Senator Robert Kelly (First appearance) (Only in flashback)
Donald Pierce (First appearance) (Only in flashback)
Conquistador (Orlando Furio) (First appearance) (Only in flashback)
Conquistador’s henchmen (First appearance) (Only in flashback)
Other Characters:

Uatu the Watcher (First appearance)
Unidentified Rigellian Recorder (First appearance)
Eternity (Mentioned)
Numerous unnamed Mutants (First appearance) (Only in flashback)
Jimmy (First appearance) (Only in flashback)
Jeremiah (First appearance) (Only in flashback)
Catholic Church (First appearance) (Only in flashback)
Human Torch (Jim Hammond) (First appearance) (Only in flashback)
The Fin (Peter Noble) (First appearance) (Only in flashback)
Blonde Phantom (Louise Grant) (First appearance) (Only in flashback)
Blazing Skull (Mark Todd) (First appearance) (Only in flashback)
The Angel (Thomas Halloway) (First appearance) (Only in flashback)
The Destroyer (Kevin Marlow) (First appearance) (Only in flashback)