Voice: Should have a Fallout ghoul style rasp. Here is one reference, and here is another! 
Higher pitch. Some nasal quality.

Early life

Franklin is a fairly old ghoul (humanoid race in Fallout), and spent most of that time as a scavenger and a failed trader. Most settlements wanted little to do with him, so he was barely scraping by, mostly surviving on his own.

One day while scavenging, he bumped into Katie. He was prepared for her to scream and run, but she asked if his old hazmat suit was a space suit, and called him out on being an alien. Surprised and taken aback, he played along. It took him a while to get into character, but once he did, he found it suited him better than being just Franklin the Ghoul. It was also fun to play make-belief with Katie, speaking in a booming voice and making up words to make a kid laugh. And when he ran into a drugged-out raider and automatically put on his alien persona, to his surprise, the raider scampered instantly.

Katie also started to trade him things to 'study', including some mushrooms from her family's farm. Franklin got the idea to crush the glowing mushrooms into a glowing paint, and touch up some of his scrap with it. To his surprise, it worked: the next time he approached a caravan, he got a slightly warmer reception and good humour instead of suspicion. The caravans took the 'alien artifacts', and he decided to take the branding more seriously. With business going better, Franklin asked Katie to stop going out of her way into dangerous spots to find salvage to trade him.

He wasn't expecting her to bring him a whole brahmin instead of salvage, but he kept playing along. Unfortunately, they lost track of time, and Katie had to run home before dark, leaving Nessie behind. She was supposed to pick the brahmin up the next day, but never showed up, leaving Franklin stranded with the thing.

Franklin is a shy and insecure guy, discouraged by the treatment he gets for being a ghoul. When he's playing as Franklin the Martian, however, he is confident and dramatic, and spends his free time thinking up ways to further the gimmick. The persona is so well entrenched now that he rarely slips up when speaking to strangers, only getting back to normal when he's shocked or afraid. He's more relaxed on it around friends.

The brand of 'alien' is important to him for both his livelihood and safety: People are better disposed to him for the alien gimmick, and he can put on a more intimidating display and bullshit convincingly to scare off raiders. It's also important as a sort of shared fiction he has running with his friend, Katie. The two spend time inventing galactic adventures for him and making up alien civilizations. Until the conclusion of the quest, Franklin doesn't realize that Katie knows he's not actually an alien, and thinks that it's the only reason she's friend's with him.

Franklin has a neutral and somewhat cynical stance on violence. He maintains character while engaged in combat, and his first thought after it's over is about looting the bodies.