Delia was companion of the journey of Giovanni and Samuel Oak, that's the only thing that is known about his past.
Delia thinks a wild Mr. Mime was their son Ash Ketchum and takes him to her house and feed him. At the end of the episode Ash appears with the clothes of a Mr. Mime, explaining the confusion of Delia. Delia likes a lot of wild Mr. Mime and ends up with him, giving it the name Mimey.