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All I did was soft reset my 3DS after every Yvetal's battle. All Yvetals I've caught gives me the following characteristics:
1. Often dozes off = 31 Hp IV
2. Likes to trash about = 31 Atk IV
3. Capable of taking hits = 31 Def IV
4. Mischievous = 31 Sp Atk IV
5. Somewhat vain = 31 Sp Def IV
6. Alert to sounds = 31 Spd IV