The extra-long Legion pilot ended with the introduction of Melanie Bird, a regal woman (played by Jean Smart, following up her awesome turn on Fargo Season 2) who promises David she can help him understand and control his powers. Is Melanie Bird from the Legion comics, though? So far, the show hasn't been a typical adaptation, and that includes adding a lot of original characters, like Melanie. There's never been an X-Men character by that name, but, just like Sydney Barrett, Legion seems to be referencing some of the comics' most famous characters in its new mentor character. According to FX, Bird is an "unconventional" psychologist, who will be an important person to help "open David’s eyes to an extraordinary new world of possibilities."
The clearest predecessor to Melanie Bird is Professor Charles Xavier, the head of Xavier's School for Gifted Youngsters. Xavier isn't a therapist, but with his psychic powers, he was able to help young mutants become heroes by helping them develop their abilities and provided a place for them to live safely, in a world where mutants are often hunted down by both bigots and scientists looking to experiment on them.
According to an interview Smart did with New Zealand newspaper Stuff, Melanie Bird has no superpowers, but the way she was able to hunt down both Syd and David with incredible precision might suggest that she has some way to sense their abilities. In the comics and film adaptations of the X-Men, Xavier uses Cerebro, a mutant finder that's powered by telepathy. Once David gets the chance to sit down with Melanie and hear why she wanted to help him, perhaps the nature of how she finds her patients will come to light.
The exciting thing about Legion adding Melanie Bird is that this is the first chance TV or film audiences have had to see a woman be the head of a team of X-Men. For example, the way Smart is playing the role feels a bit like the comics character Emma Frost — another powerful telepath who also starts her own school and program for "gifted youngsters," an often villainous counterpart to Xavier's proteges. The film versions of Emma Frost, though, pretty much just had her sitting around in lingerie (admittedly, something that is faithful to the comic books).
But the Emma Frost comparison is notable for one other reason. As explained on the Marvel Wiki, after one attack by a villainous madman, Frost falls into a coma, but survives because her "consciousness [is] still active on the material plane." That sounds a lot like how Decider described what happened to Melanie's husband before the events of Legion, writing that he "has also been trapped on an astral plane for the past 20 years."
So while Melanie Bird has never appeared in the Marvel Comics, even after a single episode there are already plenty of hints that she's going to be the newest mentor in the X-Men universe. And there's even some suggestion that she could either be a good person, like Professor X, or an ambiguous, possibly villainous leader, like Emma Frost. We'll just have to keep watching to find out.