A Jedi of the Old Republic, Obi-wan was trained by Yoda and fought in the Clone Wars.  He decided to take young Anakin Skywalker as his first pupil, but the Force strong pilot was seduced by the Dark Side of the Force and became the Dark Lord of the Sith, none other than the infamous Darth Vader.  Obi-wan tried to turn him from his evil ways, but in a final confrontation in which Vader was knocked into a geyser Obi-wan decided that his pupil was lost to the Dark Side forever.
 He discovered that Vader's wife was pregnant with twins and when they were born gave the girl to Bail Organa and the boy to his brother, Owen and his wife, Beru Lars.   He then went into hiding on Tatooine during the Jedi purge.
 When the boy, Luke, stumbled into his life again, ‘Old Ben' decided, after seeing a message from Princess Leia, that he would take Luke to Alderaan and deliver the plans to the Death Star.  When they got there, though, Alderaan was missing and the space station captured the ship.
 Ben went off alone to deactivate the tractor beam and was confronted by Vader.  A mighty duel ensued, witnessed by dozens of stormtroopers and, most importantly, by Luke.  When he had gotten Luke's attention, Obi-wan flicked up his lightsaber in a Jedi solute and allowed Vader to cleave him neatly in two.  The Jedi dissappeared, becoming one with the Force.
 Ben's spirit didn't leave Luke though, he offered help and reassurance throughout his last student's training and his confrontations with his father.  When Ben finally did have to leave Luke for good, he parted with these words, "Not the last of the old Jedi, Luke, but the first of the new."

 Famous quotes:
  Use the Force, Luke.
  The Force will be with you, always.
  Who is the more foolish, the fool or the fool who follows him?
  The Force is what gives a Jedi his power, it is an energy field created by
 all living things.
  Luke, you're going to find that many of the truths we cling to depend  greatly on our own point of view.
  Don't give in to hate, that leads to the Dark Side.
  Your insight serves you well.
  He is more machine now than man, twisted and evil.
  Stretch out with your feelings.
  If you strike me down I shall become more powerful than you can possibly  imagine.
  Run, Luke, run!

 Played by Alec Guinnes and Ewan McGreggor