In June 2012, Kaestner-Varnado participated in a WWE tryout camp and on August 18, it was announced that she had been signed to a contract. She was sent to NXT, WWE’s developmental territory, where she adopted the ring name Sasha Banks. She made her television debut on NXT on December 12, 2012, losing to Paige. Banks’ first televised win on NXT was on the January 23, 2013 episode over defeating Alicia Fox. Banks then entered a storyline where she received letters from a secret admirer, who was eventually revealed to be the evil Audrey Marie, who returned and attacked Banks on the February 20 episode of NXT due to Marie’s jealousy of Banks’ success; Marie went on to defeat Banks in a match later that episode. On the March 7 episode of NXT, Banks teamed up with The Funkadactyls (Cameron and Naomi) to defeat Marie, Alicia Fox and Aksana in six-diva tag-team match. Banks’ feud with Marie ended on the April 3 episode of NXT, when Banks teamed with Paige to defeat Marie and Summer Rae. On the June 19 episode of NXT, Banks competed in the NXT Women’s Championship Tournament to determine the inaugural champion but lost to Summer Rae in the first round.

After weeks of influence by Summer Rae to make herself relevant, on the September 11 episode of NXT, Banks was defeated by NXT Women’s Champion Paige. After the match, Paige attempted to console Banks leading to Banks attacking Paige and turning heel in the process. She wrestled her first match as a villain on the September 25 episode of NXT, defeating Bayley with Rae in Banks’ corner. After the match, Banks cut a promo with Rae and debuted a new gimmick as “The Boss of NXT”. Banks and Rae, now known as the “BFFs (Beautiful, Fierce Females)”, faced Paige and Emma on the October 16 episode of NXT, where they emerged victorious when Rae pinned Paige. On the October 23 episode of NXT, Banks defeated Emma after capitalizing on a distraction provided by Rae. On the November 13 episode of NXT, The BFFs defeated Bayley and Charlotte in a tag team match when Charlotte attacked Bayley allowing Banks to pin Bayley. After the match, Charlotte officially turned heel and aligned herself with Banks and Rae. On the December 11 episode of NXT, Banks and Rae defeated Bayley and Natalya in a tag team match. In mid-January 2014, as Rae had moved up to the main roster, the BFF’s started a rivalry with Bayley, who had aligned herself with Natalya, and lost in both singles and tag team matches. On April 6, Banks, along with Alexa Bliss and Charlotte, made an appearance at WrestleMania XXX as a part of Triple H’s entrance for his match against Daniel Bryan. In May, Banks entered a tournament for the vacant NXT Women’s Championship, defeating Bayley in the first round but losing to Natalya in the semi-finals. After a four-month absence from the show, Summer Rae returned on the June 6 episode of NXT, distracting Bayley and allowing Charlotte to get the victory. After the match, The BFFs attempted to attack Bayley, only to be chased away by Emma and Paige. This led to a six-Divas tag team match on the June 12 episode of NXT, which The BFFs lost after Bayley pinned Charlotte. On the July 3 episode of NXT, Charlotte got retribution over Bayley in a tag team match, where she and Banks defeated Bayley and Becky Lynch. After the match, Charlotte left Banks to get attacked by Bayley, with Banks officially disbanding the team in a backstage segment.

On the August 14 episode of NXT, Banks lost to Bayley in a match to determine the number one contender to the NXT Women’s Championship. At NXT TakeOver: Fatal 4-Way, Banks attacked Bayley after her title match with Charlotte, before Charlotte pulled Banks away and the two shared a staredown. Becky Lynch turned on Bayley and aligned herself with Banks on the October 23 episode of NXT, after Banks defeated Bayley in a singles match. Banks promptly began feuding with Charlotte, leading to a match between the two at NXT TakeOver: R Evolution for the NXT Women’s Championship, which Banks lost. She received a rematch on the December 25 episode of NXT, where she was once again unsuccessful. Banks made her first appearance on the main roster on the December 30 episode of Main Event, where she lost to Charlotte in a non-title match.

On January 21, 2015, Banks received another title match against Charlotte, which Banks lost after Becky Lynch attacked Charlotte, causing a disqualification as well as dissension between Banks and Lynch. On February 11, at NXT TakeOver: Rival, Banks won the NXT Women’s Championship for the first time, after pinning Charlotte in a fatal four–way match that also involved Becky Lynch and Bayley. She successfully retained the championship in a rematch against Charlotte that took place on the March 4 episode of NXT.