Maylene (Sumomo in original Japanese language versions) is the young Gym Leader of Veilstone City and specializes in Fighting-types. Her name refers to the word "melee" which relates to her fighting styles. In the game the player is required to solve a puzzle to clear the path to her, battling gym trainers, which focus on Fighting-type Pokémon along the way. Maylene's Pokémon are Meditite (Lv. 27), Machoke (Lv. 27) and Lucario (Lv. 30). She awards the Cobble Badge to trainers who defeat her. In the game, her father is seen playing at the Casino in Veilstone City. When the player talks to him he will tell him or her about his daughter who he describes as a tough one.

Maylene's first appearance is on DP66, "Lost Leader Strategy". When Ash and friends reached Veilstone City, they saw a girl there, training with a Lucario. Later, it was introduced that she was Maylene, the Veistone City Gymleader. Ash asked immediately for a battle, but Maylene refused the request because of Paul. She also appeared in DP67, "Crossing the Battle Line!", where Maylene battled Dawn using her Lucario, Meditite and Machoke. She later appeared in DP68,"A Triple Fighting Chance!", where Ash battled Maylene and won the Cobble Badge.

from wikipedia