In order to understand who Tulasi devi is, one must also understand who Vrinda devi is. In fact, Tulasi devi and Vrinda devi are one and the same.
Vrinda devi is an expansion of Radharani who lives in Goloka Vrndavana where she is in charge of planning out the details of Krishna's pastimes. She plans out every little thing, from what clothes will be worn to what flowers will bloom in the forest as Krishna traverses the paths. She also supervises Krishna's pastimes with her parrots, who fly past and relay to Vrinda devi what is going on. When Krishna is performing the rasa dance with Radharani and doesn't want to leave her in the morning, Vrinda devi sends her parrots who imitate the voice of Mother Yasoda calling out to Krishna looking for Him, and then Krishna and Radharani hurry back to their homes to pretend to have been sleeping all night. Thus, Vrinda devi is a very intimate devotee of Lord Krishna and Srimati Radharani.
However, one day a problem arose in the spiritual world. It is said that Vrinda devi seemed to get too close to Krishna, and Radharani became upset and cursed her to fall down to the material world. In the material world, Vrinda devi devoted herself wholly and solely to one goal- to attain Krishna as her husband. She began performing severe austerities with the idea that she would then be awarded some benediction. She went into the Himalayas for 10,000 years; for the first 3,000 years she subsisted only on fruits and leaves, for 2,000 years she only ate what fell near her, for 4,000 years she only took water, and for 1,000 years she subsisted on just air. After these 10,000 years Lord Brahma, the creator of this universe, came to her and asked her what she wanted. She told Brahma that the only benediction she asked for was to have Lord Krishna as her husband. However, this could not be fulfilled immediately by Brahma; first Vrinda devi would have to marry King Sankacuda, who was none other than Sudama, one of Krishna's cowherd boyfriends in Goloka Vrndavana. To make a long story short, Sankacuda was playing the role of a demon and in order to kill him, the supremely chaste Vrinda devi would have to be tricked into breaking her chastity. Lord Narayana disguised Himself as Sankacuda and tricked her into breaking her chastity, and in the mean time, Lord Siva killed Sankacuda, who then returned to Goloka Vrndavana. For Lord Narayana's "offense", Vrinda devi cursed Him to become a stone. This is the origin of Saligrama Silas. Lord Narayana told Vrinda devi that she would be a river, the Gandaki, where Saligrama Silas are found, and also take the form of the Tulasi plant.