Elizabeth (Betsy) Braddock is a character who has been appearing in Marvel UK for years. I'd probably better introduce the concept of Marvel UK, hadn't I? They were a subsidiary of Marvel who reprinted American material, and, from 1976 printed their own original stuff, starting with Captain Britain Weekly, which headlined Captain Britain, created by our very own dear Chris Claremont, with Herb Trimpe.
He's a very odd character, derived by taking the American tradition for patriotic superheroes and then just porting it directly onto an entirely different culture, and then treating the entire thing with more-or-less a straight face. Super-strong guy in a Union Jack, call him Captain Britain, give him an alliterative secret identity - Brian Braddock. For bonus points we can associate him with a kind of pseudo-Arthurian background, and a mythos that thoroughly mishmashes the Goidelic and Brythonic and then, compounding the error, conflates them with Englishness. Any other nation might be offended by this, but instead we just take over your comic book industry and try to bring it down from within.
Cap was brought into the US titles by Claremont quite quickly - appearing in a 1978 issue of Marvel Team-Up, with Spider-Man. Brian's sister Betsy, though, hasn't made the jump. Until New Mutants Annual #2, obv.
None of this material had been published in the United States, so we open with a very careful recap. Psylocke is a telepath, was recently Captain Britain for a while, but then was blinded (in Captain Britain #13). She is taken captive by Mojo, and then brainwashed and forced to appear in a TV show called "Wildways". Bobby's been captured too, and this results in the New Mutants and Captain Britain arriving at the same building in Manhattan at about the same time for a rescue mission. The ins and outs of Mojoverse are not something I'm deeply into, so to suffice it to say that she ends up liberated and hanging out at the X-Mansion.
Given this setup it's curious she doesn't end up on the New Mutants team, but the main line-up. But they did need a telepath, anyway, now that Rachel ran off. Even if she's not some kind of ninja.