Let’s review the facts:
Penny is originally from Galar
Penny’s father is described by her as being “loud and clingy”
Now where have I heard that before? Oh right:
Some additional facts:
Peony clearly gives his children similar names to his own (Take out the “y” and make it an “ia” to get Peonia). Penny is only one letter off from Peony, the only difference being that the “o” is an “n”
Peony is prone to giving children (including but not limited to his own) nicknames (“Nia” for instance)
Every single one of Penny’s non-English names reference the peony plant
Of course there is the elephant in the room; Penny is white and Peony is not.
First of all, Peony is absolutely the type of person to adopt a child and it is fully possible that Penny is not his biological daughter. However, Peony’s wife and Peonia’s mother is white. As any 8x8 punnett square will tell you, children are nowhere near guaranteed to have the skin color that is the exact middle between their parents (like Peonia seemingly is), but can in fact be nearly anywhere on the skin color spectrum, albeit with varying degrees of rarity.
(Note: the data reflected on the graph is not directly accurate to Peony and his wife. The inputs are completely different, though the outputs remain fairly accurate)
Smaller details:
One of the most common pairs of Pokémon that are often compared to each other are Pikachu and Eevee (Pokémon Yellow, Let’s Go). If Penny and Peonia are sisters, their opposing motifs make perfect sense.
Lastly, Penny gives the player character the TM for Draco Meteor (only usable on Dragon types) after defeating her. However, it is odd that she would have this TM in the first place. She couldn’t have gotten it specifically for the player because the player might not have any Dragon types, and she certainly wouldn’t have gotten it for herself; Penny exclusively used Eeveelutions (or Veevees, as she calls them), of which a Dragon type does not exist. However, there is an individual mentioned in this post who always and only has a Dragon type, that being Peonia and her Tyrantrum that she uses in Dynamax Adventures. We know Peony likes to give powerful (for lack of a better term) gifts to his daughter (See: golden expedition uniform), and he is 100% the type of dad to make sure that any gift he gives to one child, he gives to all of them.
TL;DR: Peony is Penny’s father.
