Rebecca, the mild-mannered, rare-book expert, turned out to be an ex-MI6 agent and a hard-core criminal. She had teamed-up with Curtis Hagen to get the rare diamond, clues of which were present in the 13th Chapter of Masconi Codex.
The first meeting between Neal and her was also part of Rebecca's elaborate plan. She had studied his ex-girlfriends to transform herself into his preferred ideal woman. Neal was excited about his relationship with Rebecca. He was falling in love with her. It came as a shock to him when Rebecca's real identity surfaced. He might have over-looked her con-act, but she was a killer. Rebecca had killed his new hander, Agent Shield. Also, she had killed Hagen. In the previous episode, Neal laid a trap for her, enabling Peter to arrest her.
At the end of "Short Through the Heart," Rebecca called Neal and promised that she will see him, again. She has also fallen in love with Neal. The promo of "Taking Stock" suggests that Neal does meet Rebecca. She wants to know if he still loves her. The two also talk about the diamonds, the real reason why Rebecca got close to Neal.
The sneak peek of "Taking Stock" shows her at the White Collar's interrogation room, wearing the orange prison jumpsuit. She looks confident, watchful and bereft of any emotion. Rebecca wants to know if Peter Burke does not like coming to the correction centre. He can be seen telling her that he likes to be in control of his environment, which makes the two of them. Neal is watching the proceeding from outside.
The second sneak peek provides a glimpse of Neal and Peter putting all the materials found at Rebecca's apartment in evidence boxes. To his surprise, Peter offers Neal a bottle beer at work. Peter can be seen telling Neal that they can break the rule as they are now officially off the clock.
Official Synopsis of "White Collar" Season 5 Episode 12, "Taking Stock": "When a corporate espionage case has personal ties to Neal, Peter recruits Diana from maternity leave for an undercover sting on a rogue stock trader."