The Scottish lord who joined Scotland and Britain under one crown in 1603 was Above all else James VI of Scotland. He became Lord James I of Britain and Ireland, subsequently bringing the crowns of the three realms together.
James VI of Scotland acquired the Scottish high position in 1567 at one year old, following the resignation of his mom, Mary, Sovereign of Scots. He managed Scotland for a considerable length of time before prevailing in the English-privileged position upon the passing of Sovereign Elizabeth I in 1603.
By rising to the English-privileged position, James VI of Scotland became James I of Britain and Ireland, making an individual relationship between the three realms. This occasion, known as the Association of the Crowns, didn't include a political association but instead a common ruler.
Albeit the crowns were joined together, Scotland and Britain stayed separate sovereign states with their own parliaments and general sets of laws. It was only after 1707, more than a century after the fact, that the Demonstrations of Association were passed, formally consolidating the Realm of Scotland with the Realm of Britain to shape the Realm of Extraordinary England.