Go to Hearthome City. Although first you have to beat Mayleen and Crasher Wake. After that you can challenge her. You don't have to beat Mayleen and Crasher Wake in Pokemon Platinum. -
After you get the HM and earn the Fantina in the Hearthome City Gym
She's the gym leader of Hearthome City and an user of ghost Pokemon.
In Diamond, Badge 5 can be gotten by defeating Fantina in her Gym in Hearthome City.
fantina ia a ghost type gym leader Pokemon : Drifblim Lvl.32, Gengar Lvl.34, and Mismagius Lvl.36
The Relic Badge is given to you by Gym Leader Fantina of Hearthome City (Gym #5).
There is no battlable character named Katrina in Pokemon Diamond. However, the Gym Leader Fantina can be battled in her gym after delivering the Charm to the grandmother in Celestic Town
you have to beat 2 other gym leaders before fantina appears, you have to beat wake and maylene. wake is a water gym leader and maylene is a fighting gym leader
you have to unlock celestic town
The Hearthome gym leader Fantina lets you use surf in Pokemon Diamond, Pearl and Platinum.
I think you have to go back to Hearthome city. There you can enter the gym and fight Fantina.
In hearthome city, the gym that fantina that you could not get to before. It´s a ghost type gym and when you beat it you can use surf.
the 5th Gym leader is Fantina. She uses ghost type pokemon. And if you look at the sign in front of the gym she is called the "Soulfull Dancer."