According to Hindu mythology Tulsi was actually a woman named Vrinda. She was married to demon king Jalandhar. Vrinda was a great devotee of Lord Vishnu and she prayed to Lord Vishnu for her husband to be invincible. She was also very loyal to her husband. Her loyality and prayer made Jalandhar too powerful to be defeated so no God was able to defeat him.
All Gods prayed to Vishnu to find a solution. So Vishnu disguised himself as Jalandhar and violated Vrinda. As her chastity destroyed, Jalandhar lost his power and killed by Lord Shiva.
When Vrinda came to know the truth she cursed Lord Vishnu to become a black stone (Saaligram Shila) and she burnt herself on her husband’s funeral. Lord Vishnu transformed her soul into Tulsi Plant.
As per another story Vrinda cursed Lord Vishnu to be separated from his wife.
Her cursed actually fulfilled when Vishnu turned into a black stone; the Saaligram Shila and in his incarnated form of Rama, he separated from his wife Sita.
As a blessing Lord Vishnu married her (Tulsi) in next birth as Saaligram Shila.