Piplup is one of Dawn's Pokemon.
Not much is known about Piplup. in a flashback in "Time Flies", Dawn and Ash appeared to treat Piplup as their child. When Dawn confronted Ash about how he was neglecting them both, Ash mentions how he should have stayed with May, prompting Dawn to ask who she was.
Time Flies (Flashback)
- The way Dawn and Ash argued about raising Piplup was very similar to a dysfunctional marriage.