Note: contains major spoilers for “The Mortal Instruments” and “The Infernal Devices” series.
It's a well-established theme in paranormal romance and romantic urban fantasy that true love should be forever. If one lover is mortal and the other isn't, it has to be remedied in some way: this can be as simple as a vampire “turning” his beloved, or as mind boggling as some of J.R. Ward's solutions, which usually involve divine intervention. A lover who can't or won't be turned is generally tragic backstory.
Cassandra Clare's interconnected YA Shadowhunters books are unusual in exploring relationships between mortals and immortals as a viable option, albeit not one without problems.
In The Mortal Instruments series, mortal teenager Alec falls in love with Magnus Bane, an immortal warlock who's over three centuries old. The bliss of requited love is marred by Alec's jealousy over Magnus's many previous relationships, and by his worry about growing old while Magnus stays youthful—shades of Bella in the Twilight series, but this story goes in a very different direction. With no desire to be immortal himself, Alec contemplates a spell to take away Magnus's immortality, and he realizes how wrong and foolish this would be just a little too late—Magnus finds out about the plan and dumps him, breaking both their hearts.
But the break-up is good for Alec, because he begins to see that the real nature of their conflict is Magnus's secrecy, and the inherent imbalance between them; not only is Magnus so much older, but he's Alec's first boyfriend. Being on his own gives Alec some vital self-knowledge and the confidence to assert himself:
“I've been sorry and I've understood and I've apologized and apologized and you haven't even been there. I did all that without you. So it makes me wonder what else I could do, without you.” He looked at Magnus meditatively. “It was my fault what happened. But it was your fault too. I could have learned not to care that you're immortal and I'm mortal. Everyone gets the time they get together, and no more. Maybe we're not so different that way. But you know what I can't get past? That you never tell me anything. [...] You know everything about me and I know nothing about you. That's the real problem.”
“I told you,” Magnus said softly, “on our first date that you would have to take me as I came, no questions—”
Alec waved that away. “That's not a fair thing to ask, and you know — you knew — I didn't understand enough about love then to understand that.”
By the end of the series, both have grown and gained wisdom. Alec has learned enough to not immediately jump in when Magnus asks to get back together: “I want to know about your life. And if you're not willing to tell me about it, then I shouldn't be with you. Because I know me, and I won't ever be okay with it. So I shouldn't put us both through that again.” And Magnus has realized that he has started to harden and die inside, and that he needs to share himself with Alec. With them now on equal terms, the issue of immortality is just another facet of a relationship:
“We might not have that much time,” said Alec. “I'll get old and I'll die. But I promise I won't leave you until then. It's the only promise I can make.”
“A lot of Shadowhunters don't get old,” Magnus said. Alec could feel the thrum of his pulse. It was strange, Magnus like this, without the words that usually came to him so easily.
Alec turned around in Magnus's embrace so that they faced each other, taking in all the details that he never got tired of: the sharp bones of Magnus's face, the gold-green of his eyes, the mouth that always seemed about to smile, though he looked worried now. “Even if it were just days, I would want to spend them all with you. Does that mean anything?”
“Yes,” Magnus said. “It means that from now on we make every day matter.”
In The Infernal Devices, relationships between immortals and mortals also turn the paranormal “one fated mate” trope on its ear. Like Magnus, Tessa Gray is an immortal warlock, though one who's only just started her life. Her introduction to the Victorian Shadowhunter world includes loving and being loved by two very different mortal boys: Will is moody, rebellious, and constantly pushes people away, Jem is sensitive, gentle, and slowly dying. The story practically writes itself, doesn't it?
Well no, actually it doesn't. Through a complex series of magical events that occur throughout both of the series, Tessa gets to be with both men at different parts of a long, often lonely life, which includes the painful realities of outliving Will and their children. When she and Jem are reunited, in the present day, she's not without fear of once again sharing her life with someone who can't be with her forever:
Jem was mortal now. He would grow old like Will, and like Will he would die, and she did not know if she could bear it again.
Most people are lucky to have even one great love in their life. You have found two.
Like Magnus and Alec, Tessa's loves ultimately meet her on an equal basis. Will and Tessa are about the same age and they grow old together, although she always looks youthful. When she and Jem are reunited, both are physically about the same as when they first met, but both have experienced many years of living:
When she looked more deeply, she saw a man — the long years of pain and wisdom at the backs of his eyes; even the way he moved spoke of the care of quiet sacrifice.
Tessa's love for Jem takes nothing away from her love for Will; in fact, they can share memories of him. And Jem knows his beloved friend would approve: “He is happy, Tessa. It gives him joy to see us together, as it always gave me joy to see the two of you.”
The Shadowhunters world is a diverse one, with characters of different races, sexual orientations, religions, and abilities. It also stands out, especially in Young Adult fantasy, as a world which celebrates the importance of love but in which you don't have to change an essential aspect of your being to keep it, where the loss of a love can be survived, and where it's possible to have more than one true love.