Ralts
Ralts: In the Abandoned Railnet, the part where you have to connect the tracks there are darkened sections of wall. These walls can be rock smashed down and one of them will contain a corrupted pokeball. Bring the pokeball to the pokeball store and there will be a npc in a white coat. He is a ball designer and after some dialogue referencing Missingno he will ask for magnet powder bought on 7th street to fix the pokeball. Bring the magnet powder to him and you'll get a ralts.
Where is that pokeball store ?