Lopez made her initial WWE debut in 2018 during the November 14 episode of NXT, losing to Lacey Evans. Lopez returned the following year in 2019 during the April 9 episode of SmackDown Live. During the program, she was simply acknowledged as Karissa, competing as one-half of a local tag team combination with Kris as "The Brooklyn Belles". They lost a tag title match against the WWE Women's Tag Team Champions The IIconics.
She made her NXT debut on the June 22, 2021 episode, under the name Elektra Lopez in a match against Franky Monet. She went on to win her next two singles matches during the September episodes of NXT, defeating Anna Scheer and B-Fab, the latter of whom she defeated in a no disqualification match. She would soon join Legado del Fantasma's Santos Escobar, Joaquin Wilde and Raul Mendoza, and would remain with the group until the August 23, 2022 episode of NXT when the stable, except Lopez, left the brand. When the group was called up to SmackDown, Lopez was replaced by Zelina Vega as the stable's manager.
At NXT New Year's Evil on January 10, 2023, Lopez participated in a twenty-woman battle royal for a chance at Roxanne Perez's NXT Women's Championship at NXT Vengeance Day, eliminating Thea Hail before she was eliminated by Nikkita Lyons. She first teamed up with Lola Vice on the April 14 episode of NXT Level Up, losing to Katana Chance and Kayden Carter. On the June 13 episode of NXT, Vice officially allied herself with Lopez. Their alliance ended on the January 16, 2024 edition of NXT, when Vice eliminated her from a number one contender's battle royal and the two then fought to the back. On the January 23 episode of NXT, Lopez confronted Vice during a Supernova Sessions segment and proceeded to attack her, thus setting up a match for the following week. On the January 26 episode of SmackDown, Lopez interfered in a match between Santos Escobar and Carlito, helping Escobar pick up the win and re-joining Legado del Fantasma on the main roster in the process.