Starlow is part of a race called star sprites. They’re these little orbs with faces and feet (shoes to be exact), along with a star floating above their head. She’s good friends with Mario and Luigi, although a little bit pushy and (though playfully) rude towards the latter. Right now she isn’t as annoying as she is in the games. At first she thought she wasn’t in Brothership and was pretty shaken up by it, but thanks to the overview trailer showing that she IS in the game, she’s alright now.