Shelby

Mr. Shelby was a British citizen living in London in the Victorian era.

Suffering from paralyzing timidity, Shelby started visiting Trevor Alton's office, a local doctor. Alton developed an experimental drug in powder form, which seemed to be having great success on Shelby, who started feeling like an entirely new person, being more invigorated and outgoing than he had ever been before.

However, due to the negative side effects Alton himself experienced from the drug, Shelby's treatment was immediately ended without any explanation, leaving Shelby disappointed and angered.

When a fellow patient of Dr. Alton, Prudence Browne, was found murdered in a nearby park, Shelby was briefly interrogated by the Assassins Jacob and Evie Frye, who quickly deduced that Trevor Alton had committed the murder because of a personality-disorder caused by the experimental medicine.

Reference

 * Assassin's Creed: Syndicate - The Dreadful Crimes