In the Pokmon Emerald version, Golbat evolves into Crobat when it levels up while holding a Friendship item.
Friendship items are held items that increase the friendship level between the Pokmon and its trainer. The higher the friendship level, the more powerful the Pokmon’s moves will be. There are several different Friendship items that can be used to evolve Golbat, including the Soothe Bell, the Friendship Band, and the Squirtbottle.