When his twin brother is murdered, a law student discovers that he was moonlighting as a vigilante; donning his brother’s mask, he takes on the criminal underworld to hunt down his brother’s killers.

Or instead of a physical handicap -- it could be psychological. ?His brother got in such deep and finally fatal trouble because he was extrovert, the more adventurous one, the one who loved to take risks, live on the bleeding edge. ?While his brother was the introvert, the one who always avoided risks, played it safe.

Now the the brother has to "put on" his brother's temperament as well as his clothing and personality. ?And, of course, initially it will be a very uncomfortable fit.

Or instead of a physical handicap -- it could be psychological. ?His brother got in such deep and finally fatal trouble because he was extrovert, the more adventurous one, the one who loved to take risks, live on the bleeding edge. ?While his brother was the introvert, the one who always avoided risks, played it safe.

Now the the brother has to "put on" his brother's temperament as well as his clothing and personality. ?And, of course, initially it will be a very uncomfortable fit.

Agreed with the above.

I'dd add that giving him a handicap of some sort would likely make it more interesting and add to his obstacle. A gamy leg, one arm, blind or deaf?- maybe that could be your hook. Zatoichi a blind samurai, made for an incredible MC and a great film - though not really blind he chooses to be, which probably makes him even greater.