Jessie caught Seviper in A Tail with a Twist after initially being impressed with its sneaky and aggressive demeanor. It had defeated Ash's Treecko earlier in the episode, which is why Jessie wanted it so much. Although Seviper was intelligent, and outsmarted Team Rocket's multiple attempts to ensure its capture, Jessie caught Seviper after it had bitten off a chunk of her hair, mangling poor Seviper with her "Fury Swipes" and "Mega Kick".
Seviper is Jessie's primary battling Pokémon, often seen battling alongside James's Cacnea in Hoenn and later James's Carnivine in Sinnoh. While shown to be fairly powerful before being caught by Jessie, after it fell into her command, it suffered the inevitable fate that all Team Rocket Pokémon face- defeat by Ash or another protagonist.
Seviper has also entered a few Contests. In Mademoiselle Jessica's Contest debut, in All Things Bright and Beautifly!, Seviper was Jessie's chosen Pokémon to enter the Contest, but miserably failed to pass the Appeal Round, as Jessie didn't prepare herself to the Contest, and ordered Seviper to use attacks that Seviper can't learn, such as Blaze Kick. She is then seen apologizing to it, meaning she does care about Seviper.
In ZigZag Zangoose!, Seviper was send out in an attempt to capture Pikachu. However, Seviper noticed Nicholai's Zangoose and the two of them start to fight, ignoring everyone else around them. They both fall into a river and Seviper knocks Zangoose unconscious with its Poison Tail. Just as Seviper is about to finish Zangoose off, Ash sends out Corphish. Corphish uses Bubble Beam to attack Seviper, allowing Zangoose to escape. Team Rocket later uses a machine to amplify Seviper's voice to draw Zangoose to them. Seviper and Zangoose continue their fight while everyone else fights over Pikachu. Zangoose ends up beating Seviper this time and Team Rocket is sent blasting off.
In The Bicker the Better, Seviper was used in a Double Battle along with Cacnea against Oscar and Andi and their Nidoking and Nidoqueen. Seviper ends up using Wrap on Cacnea, which causes Jessie and James to argue, allowing Oscar and Andi to take the win with dual Hyper Beam attacks, sending Team Rocket flying. Team Rocket later encounters Ash and May and agree to do a tag battle with them, with Ash using Corphish and May using Skitty. Cacnea and Corphish end up working well together and begin to get an advantage over the Seviper and Skitty. Jessie soon sends out Dustox to help win and the battle quickly grows out of control when attacks start to fly undirected. Meowth then takes the opportunity to try to capture Pikachu and has Seviper and Cacnea use Haze and Sandstorm. Pikachu is rescued and Team Rocket is sent blasting off.
In Queen of the Serpentine!, Seviper was used to battle against Pike Queen Lucy and her Seviper. Barbara points out that Jessie's Seviper is weaker than Lucys, which infuriates Jessie and Seviper. Both Sevipers collide with their Poison Tails and true to Barbara's word, Lucy's Seviper's Poison Tail is more powerful and sends Jessie's Seviper flying. Lucy's Seviper finished it off with a Flamethrower, blasting Team Rocket off.
In Settling a Not-So-Old Score!, Jessilina used Seviper in the Appeal Round. Not much of the Appeal was seen, because while Seviper was performing it, Dawn was thinking about her Pachirisu's Appeal, focusing the attention on her, not on Jessie. However, Kenny stated that Seviper was being quite good. The appeal ends with Jessie ordering Seviper to perform Wrap and Bite on Jessie herself, and thus, amazing the crowd.
In the episode A Stand-Up Sit-Down!, Jessilina decided to face Dawn to "practice" for the Hearthome City Contest. However, it was Zoey who accepted the challenge. Jessie used the combination of Seviper and Dustox and her idea was to make Dustox use Poison Sting and then to make Seviper use Poison Tail on Dustox's Poison Sting, making it spin and added extra-power to it. Brock even said that that was a smart combination. However, that didn't work as Zoey's Glameow countered with Iron Tail, making the combination going to other side around and then Zoey's Shellos used Mud Bomb, breaking the combination of Poison Sting and Poison Tail. Seviper and Dustox both were then easily defeated by Zoey's Glameow.
In Dawn's Early Night!, Seviper was used along with Dustox in the Battle Round of the Hearthome City Pokémon Contest. They were briefly seen battling against Nando's Kricketune and Roselia in the semifinals. The battle ended after Roselia and Kricketune used Petal Dance and X-Scissor, respectively, to deplete their points.
In Old Rivals, New Tricks!, Seviper was Jessilina's choice for the second round of the Sandalstraw Contest. It battled against a Gastrodon in the first match and managed to win by using its Wrap attack, reaching the semifinals. There, it went up against Dawn's Ambipom. Despite putting up a good fight, it was eventually defeated, resulting in Jessilina's elimination from the event.
Since Jessie has caught Yanmega, Seviper wasn't seen in as many battles as before. It was also used as Team Rocket's escapist rogue to blanket the area in Haze, replacing James's Weezing's duty to steal innocent Pokémon under the shrouded premises.
In Yes, in Dee Dee it's Dawn! and Playing the Performance Encore, Jessie used Seviper for the Daybreak Town Contest. In the appeals round, it made an entrance by coming out of its Poké Ball, which was placed inside James's Carnivine's mouth. In the next episode, it battled alongside Yanmega against Ursula and her Plusle and Minun. Seviper and Yanmega were able to combine their attacks well, creating a Haze-Sonic Boom and Poison-Silver Wind. However, Plusle and Minun were able to defeat them using their Spark and Helping Hand attacks, knocking Jessie out of the competition.
In Teaching the Student Teacher!, Jessie used it this time for the Neighborly Town Contest, where it was used for the appeals round, aiding her win her fourth Ribbon. In Dawn of a Royal Day!, Jessie used again for the Arrowroot Town Contest in the battle round, where it fought Princess Salvia (disguised as Dawn) and her Togekiss. After a fierce battle it lost, but Salvia gave her the Ribbon for the performance she gave during their battle, allowing her to enter in the Sinnoh Grand Festival.
In the Grand Festival, Jessie used Seviper multiple times, including the Appeals Round, a battle with Yanmega as well as a battle alongside Carnivine. However, Jessie lost her final battle to Dawn.
Following the Team Rocket trio's journey in Sinnoh, Jessie was forced to leave Seviper at Team Rocket's headquarters as it is not a Pokémon native in the Unova region and thus would attract unwanted attention to the trio.