Raiden is completely mute and quite monotone with expressions and emotions. 

He grew up in a lab where he was used as a live experiment, however when one of the employees who were looking after him decided to flee, she took along the -at the time- young teen, not wanting them to harm him any more than they already had. 

Raiden seemingly has an electric and tingling air around him, and thunderstorms seem to often follow him wherever he goes, which is why the villagefs I'm the area now thinks of him as a walking omen and a disaster.