Awh, thank you so much!! There's actually a couple different stories for all of them.

For example; Silver is a failed lab experiment by the scientists, however Spark is specially trained by a Rocket Admin specifically for battling, same with Boss and Dusk. Meanwhile Mitzi is a successful clone and the team rocket boss is looking to sell her to the highest bidder. Lastly, Mizu, who was caught and is owned by a grunt who is trying to get higher up in the organization.