Dawn was a former travelling companion of Ash who accompanied him through the Sinnoh region. She was voiced by LiniumConcolor.
Dawn appeared in a flashback in "Time Flies". After comparing her to Dawn, Iris demanded to know who she was, prompting Ash to tell another story. Dawn is seen arguing with Ash over his neglectful behavior. It is revealed that Dawn was left to "raise their Piplup alone". This caused Ash to state that he should have stayed with May. Dawn then demanded to know who May was.
Time Flies (flashback)
Mayhem Big and 'Mall (fashback; cameo)