Sure, I'll bite. Note, I have absolutely no idea about anything in XCOM and have never played the games, but I made something from the limited information I have.

[hider=Milla Duchene][i]Name:[/i] Milla Duchene
[i]Nickname:[/i] Duchess. Because of the resemblance in her last name. 
[i]Gender:[/i] Female

[i]Backstory:[/i] Born in 2005, she was ten years old when the alien invasion happened. Her family were a bunch of farmers, not much that they could do in the face of such power. Her father was a member of the original XCOM, but he died when the base was destroyed. Milla knew this as she was growing up, but due to the lack of things that she could do about it she and the rest of her family allowed themselves to be subdued. However, just in case, Milla secretly trained herself, kept herself fit and used any means necessary to learn how to operate a weapon. She may have joined ADVENT, depending on if that was composed of humans or not. Because, anything where she could fight was good for her.

Then, when XCOM reformed, Milla jumped at the chance to once again combat the alien threat in place of her father, hoping to be able to succeed where he failed and live to tell the tale.

[i]General appearance:[/i] Mildly tall, serious look upon her face. I’d put her with short, black hair, probably cut it because long hair would get in the way while fighting. She could potentially have suffered a scar here or there during her physical years, though probably wouldn’t have gotten a tattoo unless it was a common thing, that’d you be left out if you didn’t get one. I imagine her with rather sharp features, yepp.

[i]Attitude:[/i] Twitchy, would probably be the best description. She’s somewhat nervous about fighting these creatures that she knows have beaten them before, and might be easily startled. But, she’s determined to slay these bastards, and she’s a thorough professional at what she does.

[i]Class:[/i] Ranger. She wants to see the whites in their eyes… Er, or any equivalent which might be relevant among aliens.
[i]Subclass:[/i] Assault. She’d have it no other way.[/hider]