Rachel Alucard is a vampire who is literally thousands of years old. Through some unexplained power, Rachel is the only character in the story who maintains a full memory of events throughout the time loops, even though all other characters have their histories reset each time. As a result of seeing the same events repeat themselves over and over again, she usually wanders around with a bored demeanor, occasionally picking a fight with one of the main characters simply to amuse herself in between the time loops. She has a butler named Valkenhayn under her employ, who is rumored to be one of the Six Heroes himself. She also walks around with two familiars: a pudgy, bat-like thing named Gii, and a morphing figure with a cat face named Nago.
As the current head of the Alucard family, Rachel sees it as her duty to find a way to stop Terumi's evil machinations as she manipulates some of the main characters in order to find a way to break the time loop. Rachel was the one who turned a wounded Jin into Hakumen using the SUSANOOH Unit and turned Ragna into a (half) vampire. Apparently, she even killed Terumi sometime in the past, but he just came back as a smiling ghost and mocks Rachel for being a "shitty vampire," making the situation worse. Though she maintains her knowledge of each time loop, she seems either unwilling or unable to tell the truth to anyone outside of cryptic riddles, at least until the True Ending.
When the True Ending rolls around, Rachel mentions that Noel was the true heir to the knowledge of the Azure Grimoire instead of Ragna, and decides to take action to end the time loop by activating the Tsukuyomi Unit, an ultimate defensive shield that saves Kagutsuchi from destruction. However, while she's away, Terumi (in the body of Hazama) makes Noel turn her gaze upon him, and gains the knowledge of her Grimoire through that (don't ask how this is possible), and by the time Rachel returns to stop her it's already too late. Terumi seems to think that Rachel is the only one capable of beating him, so he disappears while Rachel tries to find another way to defeat him once and for all.
In the game itself, Rachel is difficult to master as she requires a little planning to effectively use. With the help of her familiars, she can summon electric frogs, pumpkins, and even lightning rods out of thin air to attack her enemy. Her Drive ability allows her to briefly manipulate the direction of the wind, which she can use to either misdirect enemy attacks or propel herself directly at the enemy for bigger combos. However, Rachel has little defense, and will not last long in hand-to-hand combat with a stronger fighter, so smart Rachel players will do whatever they can to keep their enemies at a distance, mixed with occasional hit-and-run attacks.