Pokémon Ruby and Sapphire/Mossdeep City

Mossdeep City is where you find the seventh Badge.

Space Center
Go inside the Space Center to get the Sun Stone, which will allow you to evolve certain Pokémon.

Pokémon Gym #7
There are Psychic-Type Pokémon in this Gym.
 * Prize: Mind Badge
 * Gym Leaders: Tate and Liza

Psychic Preston
 * Lv. 37

Psychic Maura
 * Lv. 36
 * Lv. 36

Psychic Samantha
 * Lv. 37

Psychic Fritz
 * Lv. 35
 * Lv. 35
 * Lv. 35

Psychic Virgil
 * Lv. 35
 * Lv. 36

Gym Leaders Tate and Liza (simultaneous)
 * Lv. 42
 * Lv. 42

People

 * Tate & Liza
 * Steven Stone
 * Nurse Joy