An Implied Character is someone who had been mentioned in the series, but never made an actual appearance.
Mika was a girl mentioned by Pop in Ep. 15 of Season 1 when she explained why she arrived home late at the beginning of the episode. Since none of Pop's known classmates go under the name, Mika remains unidentified.
Mika was renamed 'Millie' in the dub.
Witch Queen's Mother
Light Novel Series
During the light novel series, it was stated that Onpu was currently in a relationship with a boy. However it is unknown if his identity was revealed or not.
In Episode 4 of the dub, Mirabelle states that she never told her dad where the dog hid the TV remote. The currently status of the dog remains unknown, most likely possibilities being it has either passed away or is currently living with Mirabelle's mother.