"There are," Raziel immediately answered, before chuckling and shaking his head, "but I may have to think of what is the best way to go about it. The human pledging fealty is one way to do it, off the top of my head, but that isn't what you desire. You want her to be your equal, want her to spend eternity with you. Outside of the more dark methods, nothing outright jumps out at me." He paused to consider, tapping his nails over his jaw, considering it.
After a minute or two, he spoke again, "Now, there is a chance that you could give to her a piece of yourself. Not in the physical sense, though to signify it physically would be rather appropriate. In war times, a woman would give to her soldier husband a lock of her hair to remind him of her, to protect him from straying and from harm, and the soldier would reciprocate in kind, to remind her of him, to keep her safe while he was off fighting. The lock of hair would be kept in a locket, worn close to their hearts. It was superstition that made the sentiment strong. But in that sentiment, there is my inspiration. A lock for a lock, a heart for a heart, a piece of one for the other. You could give her a part of you to make her live for as long as she desired, a part of you that shone through in her. It wouldn't need to be much, but it would... it could possibly work for what you desire. A spiritual exchange, a bonding rite by transference."