This Vileplume is a grass/poison-type Pokémon owned by Erika.
Red, Blue & Green arc
Vileplume debuted when Red visited Erika's Gym. After retrieving Eevee, Red challenged Erika to a Gym Battle. Erika used Vileplume to attack Pika, pushing him to the limits. When Pika used Substitute to protect Eevee, Erika canceled the battle and gave Red the Rainbow Badge.
Later, Erika appeared in front of Yellow with Vileplume and Tangela, giving her advice on finding Red. Also, when the Kanto cities were under attack of Elite Four's armies, Erika used Vileplume and Tangela to fight back against the Shellder army.