You need to go to lake Acuity and surf into the cavern into the middle near Snowpoint city to find Uxie after you have caught or killed Palkia (i accidentally made it faint! Oh yeah it doesn't die it just goes back to its realm of time), but you need to fight galactic commanders Saturn and Mars then the boss Cyrus. You can find it at the top of Mt. Coronet at spear pillar (palkia)
First buy lots i mean LOTS of ultra balls (like 40, i needed to use 15 to catch my Uxie!!!) because Uxie is hard to catch.
SAVE IT BEFORE YOU FIGHT IT! Because once it has fainted it will never come back(there is only one in the whole game so if it faints you can turn your ds off off so you get another few chances) get its hp down to red, i think the easiest way to do this is use the moves take down and ember 2-3 times(be careful as this last move could give the Uxie a burn) bearing in mind i had a level 55 Rapidash to do it with. Effects may vary depending on the Pokemon and its level.
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memmemem whatever that means,#146 is Uxie
you cant get it in emerald, its only in diamond, pearl, and platinum
You can't capture Uxie more than once on this game, but you can own more than one Uxie if it is traded to you.
Pokemon 146 in the national pokedex is moltress and Pokemon 146 in the sinnoh pokedex is uxie. have fun!
you need azelf,uxie,mesprit to open the cave after beating the Pokemon league