These are the following episodes in which Cynthia has appeared:
Diamond Pearl Series
An Elite Meet and Greet (mentioned, not her first proper appearance) DP Episode 35
Top Down Training (properly made her first appearance) DP Episode 40
Losing Its Lustrous! DP Episode 96
Double Team Turnover DP Episode 97
Aiding the Enemy DP Episode 100
The Needs of Three! DP Episode 151
The Battle Finale of Legend! DP Episode 152
An Old Family Blend! DP Episode 182
Memories are Made of Bliss! DP Episode 191
Black and White Series
All for the Love of Meloetta! BW Episode 85
Piplup, Pansage, and a Meeting of the Times! BW Episode 86
Jostling for the Junior Cup! BW Episode 90
Battling Authority Once Again! BW Episode 91
Ash, Iris and Trip: Then There Were Three! BW Episode 92
Goodbye, Junior Cup - Hello Adventure! BW Episode 93
Meloetta and the Undersea Temple! BW Episode 96
Unova's Survival Crisis! BW Episode 97
Beauties Battling for Pride and Prestige! BW Episode 98
Source
Let me know if I missed any. Hope this helps.
