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GameFAQs: Final Fantasy XII (PS2) Ultimate Equipment FAQ by bcs4981

And no, you can't get any Genji pieces if you missed stealing from him. The thing is, after every cutscene (where he draws a sword) you can steal another item.
21-40% HP first encounter: Genji Shield
1-20% HP first encounter: Genji Gloves (this piece of equipment is pure ownage with high combo rate weapons such as Masamune or Turnesoul)
21-40% HP second encounter: Genji Helmet
1-20% HP second encounter: Genji Armor (I think this armor gives you 4x counter attack rates with Battle Harness)