Manga Like Gas-tou Norainu Tanteidan

The setting is London at the end of the 19th century. Rui, a vagrant boy, meets Sherlock Holmes, a consulting detective who has just started his own practice, after a murder case. Although he harbors hatred for Holmes, who doesn't pay attention to anything other than upper-class cases, Rui decides to work as Holmes' "dog" in order to gain the ability to protect vagrants.