Although originally conceived as a minor character, Kuryakin became an indispensable part of the show, achieving co-star status with the show's lead, Napoleon Solo.
McCallum's blond good looks and the enigmatic persona he created for the character garnered him a huge following of female fans.
Such was the popular hysteria surrounding him that he was referred to in newspaper reports at the time as "the blond Beatle" or "the fifth Beatle".
While playing Kuryakin, McCallum received more fan mail than any other actor in the history of MGM.

Much of the character's appeal was based on what was ambiguous and enigmatic about him.
When an acute reaction to penicillin hospitalized him in the early days of filming the series, McCallum took the opportunity to give serious thought to how he might flesh out what was, at that stage, a sketchy peripheral character.
The approach he hit upon was to build a persona based on ambiguity and enigma, hiding, rather than revealing, aspects of the character's background and personality.
McCallum summed up the character in commenting "No one knows what Illya Kuryakin does when he goes home at night."
Although originally conceived as a minor character, Kuryakin became an indispensable part of the show, achieving co-star status with the show's lead, Napoleon Solo. McCallum's blond good looks and the enigmatic persona he created for the character garnered him a huge following of female fans. Such was the popular hysteria surrounding him that he was referred to in newspaper reports at the time as "the blond Beatle" or "the fifth Beatle". While playing Kuryakin, McCallum received more fan mail than any other actor in the history of MGM.
Much of the character's appeal was based on what was ambiguous and enigmatic about him. When an acute reaction to penicillin hospitalized him in the early days of filming the series, McCallum took the opportunity to give serious thought to how he might flesh out what was, at that stage, a sketchy peripheral character. The approach he hit upon was to build a persona based on ambiguity and enigma, hiding, rather than revealing, aspects of the character's background and personality. McCallum summed up the character in commenting "No one knows what Illya Kuryakin does when he goes home at night."
Illya Kuryakin