She is the Navigator of the phantom thieves team. Much like Morgana being the staple "leader," Futaba is very much the brains of the team. Being a young 15 years of age, younger than most of the team, she very much so is mentally the age of a mad scientist, more than einstein. Futaba has been on the internet for a long time and has a hacker alter ego known as "Alibaba" which formally was Medjed. She hacks for justice, and interest for herself of course, but she is by no means selfish. Around her friends and team, she is childish, cunning, and what we call today "a meme machine." However, when met with new people and in the outside world alone, she can be very timid and shy. Think of a Redditor in its true form, and Futaba could very well be a result. Gamer girl. 

For a voice? Think of a gremlin girl who games. That one overwatch girl that makes fun of you for how bad you are at a game, but doesn't know what to do when you clap back at her and she gets all timid. A smartass, a childish teen, fast thinking. 

(High voice)