Pokémon Trading Card Game/Grass Club

The Grass Club is an all-girls club of Grass type Pokémon decks. It's a clear analogue to the Celadon City Gym in the mainline games. The club leader Nikki isn't here right now, and her club members won't tell you where she is until you defeat all of them.

Club Master: Nikki
Once you have defeated all three of the other members of the club, they will tell you that Nikki has gone up to Ishihara's house. The house in the northwest of the map will now be called Ishihara's House and you can go there to find Nikki. She says she was doing research and heads back to the club to be able to duel you. She will be at the north end of the room Kristen and Heather are in.

A victory over Nikki results in you obtaining the Grass Medal.

Card Trades
There is a girl in the lounge of the club who wants an Oddish Lv.8 card. She'll trade you a Vileplume Lv.35 card for it. You can get Vileplume in the same Mystery booster pack that you can also find the Oddish in so this isn't a unique trade you absolutely must do, but it is a nice trade of a common for a rare if you're struggling to find a Vileplume.