No, legendary Pokemon only appear once. If you defeated Registeel, then he's gone from the game for good. Hopefully, you saved before you battled him.
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It disappears. If you haven't saved before he fainted, turn off the game and turn it back on.
you cant unless u tade but someone else says if you defeat the elite four they come back even if u defeat them i dont know if its true or not hope it helped
You can obtain a Registeel by trading it from Ruby, Sapphire, and Emerald.
You have to beat the elite four and then look for him in the same spot he was when he fainted
Registeel is unobtainable from Pokemon Diamond and Pearl. You have to migrate it from Pokemon Ruby, Sapphire, or Emerald.