You don't have to see her afterwards.
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 can find her at the Contest Hall.
in platinum she is in the Pokemon contest center
You unlock the HM move Surf when you beat Fantina. Before that, you go to celestic town and you will meet Cynthia's grandmother in the ruins she will give you surf.
Beat maylene and wake first.
Beat Fantina in Hearthome City.
In the Hospital. Because she has diarhea.
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.
After you get the HM and earn the Fantina in the Hearthome City Gym
fantina ia a ghost type gym leader Pokemon : Drifblim Lvl.32, Gengar Lvl.34, and Mismagius Lvl.36
you go back to Hearthome city to see Fantina cos she will be there