He had no choice but to lull her into this "imprinting" process. She had drank some of his blood but she was not one of his kind. In the future, she would be able to dismiss the illusion and come in and out of the his hidden valley, as she pleased.
Even though, he saved her, he was not willing to endanger his whole clan for a stranger. At least by "imprinting" her, she would not be able to cause him any harm. Fate had linked them together and she would never be able to turn against him now.
This "imprinting" process was quite peculiar. Only the Master of the Valley was able to "imprint". The "imprinting" skill was imparted from the previous Master of the Valley to the next one, usually from father to son. In the "imprinting" process, the "imprinter would lose some of his lifeforce to sustain and fuel the contract. It was a desperate, last recourse means to receive and guaranty the absolute loyalty of a person. Unless it was necessary, the Master of the Valley would definitely not use that secret skill.
Once a party was "imprinted", it was like an invisible bond, an unbreakable contract.
There were different levels of "imprinting", depending on the symbol chosen and the amount of lifeforce that was spent, but the concept was the same: allows for the "imprinter" to have power over the life of the "imprinted".
The "imprinting" he chose allowed for him to know her intention, location and health status at any time. He would also be able to restrict and order her, if it was needed. However, if she was to face any danger, he would be compelled to save her. Any harm that would befall on her, he would also feel it. If she was to die, he would not die along but would receive an equal punishment and if he was to die, she would also not die but would either become mindless or crazy.
He could have chosen another "imprinting" where the consequences would have been less damageable on him, but this one was the only way for him to restrict her actions. By using this symbol, he was truly her Master now.
He found the process of "imprinting" and his duties as "imprinter" very troublesome, but he needed to ensure she would never harm the Valley, so he resolved himself to use it. He had to spent 1/10 of his lifeforce for this contract. Besides "imprinting", the only other safe option would be to kill her. However, he was very unwilling to do it: he did not brought her back from death's door just to send her crossing the Styx river...
As to why he was unwilling, he really had no clear idea of why. He found her very interesting and believed she could relieve some of his boredom. Was that a good reason enough?
Of course, Lily had no idea what this "imprinting" process was about. She just found it strange and worrying. She knew she would have to research this once the memory reliving was complete.