Rakefine is Sasha's great uncle and benefactor, and a member of the Harlequins.
Sasha has a poor view of Rakefine: she says he's "just a Racket that’s spelt his — that put his money in a different place."
According to Sasha in episode 124, she was sent to live with Rakefine in Upper London for a couple of years. Earlier, in episode 11, Sasha says that Barret "sold" her to someone in Upper London; Barret says that he sent her to Upper London to "formalis[e] [her] education." In the pre-season 3 recap episode, Lydia says that she was was sent to Rakefine because of "a mysterious inheritance."
Sasha considered her arrangement with Rakefine a "prison." She wasn't told much about what was going on, but was forced to become a "posh lady," to talk to people, and to wear long skirts; she couldn't climb on roofs. At some point during the time she was living with Rakefine, she received a ring from Barret that she couldn't remove. Sasha tried to escape several times, but every time she left, people would tell her she needed to go back and threaten the people that she liked.
According to the pre-s3 recap, Rakefine was "training her for something." He hired several private tutors for Sasha, including Eldarion. According to Einstein, Eldarion thought Rakefine was a "cool guy" and spent increasing amounts of time away from her main job at Prague University. At some point during this period, Sasha was listening in on a conversation between Eldarion and Rakefine and heard them mention the Harlequins (this incident is described in episode 61.) This was one of the few times that Rakefine ever got annoyed at her; according to Alex, Sasha "never really saw him angry".
Sasha and Eldarion didn't get along and after a while, Eldarion left. Rakefine suggested that Sasha get a job with Bi Ming Gusset, who refers to Rakefine in episode 18 as his "late friend and Sasha’s benevolent benefactor." Sasha's relationship to Rakefine during her employment with Bi Ming isn't clear.
According to Einstein in episode 124, Rakefine was also the head of a chapter of the Harlequins. In episode 61, Earhart mentions that Rakefine "was the big-picture guy" for the Harlequins. Bi Ming mentions in episode 18 that Rakefine wore a spades signet ring, signalling his affiliation with the Harlequins.
Rakefine seems to have died sometime before the beginning of the campaign, as Bi Ming refers to him as his "late friend." Before he died, Rakefine left Bi Ming a list of names to give to Sasha, which he passes on to her in episode 18; the party recognizes some of them, such as:
It is possible that Rakefine is the person for whose "will" Barret is looking in the prologue: Sasha says in episode 11 that she could "go back to the house" to look for the will for Barret, since she might "know where he was keeping stuff."
It's also possible that Rakefine's first or last name is Martev: her characterization of Martev as Barret "but in a better suit" (episode 11) is similar to her characterization of Rakefine as a Racket "that put his money in a different place" (episode 124). Similarly, she says that Martev "never told [her] anything" (episode 11) and that during her time with Rakefine, "they never tell [her] why" they were teaching her things (episode 124).