Jessie's Arbok evolved from her Ekans as she was complaining that she needed a stronger and more evolved Pokémon that could help her during battles. Moments later it evolved and she gave it a fitting introduction to Ash and his friends in the Diglett forest. It's first battle didn't go well as it got outnumbered by the Diglett and Dugtrio.
While in the Hoeun region, Arbok became attached to a group of Ekans that had been captured by a Pokémon Poacher. It was able to help them escape from the poacher and Jessie saw how much it liked those Ekans and told it to stay there and watch over them. Initially, Arbok didn't want to leave but Jessie was insistent and Arbok was released.