Sarah J Maas is able to churn out male love interests at an uncanny rate.
Here's how you can write a love interest like Maas.
He's tall, dark and handsome...
Rhys, Cassian, Azriel, Dorian, Chaol... need I say more?
...with a mysterious, dark past
Whether it's an abusive father, a murder they committed in secret, a hidden heritage, forbidden romance, a dead family member, your romantic male interest must have scars from his past which only your heroine can heal.
He's powerful and dangerous...
Make sure that he is dangerous—he can kill everyone around him if he wanted to. He'll be stronger than those around him, and often have a special magical power which sets him apart.
...but will never hurt your heroine...
Despite his power, make sure that he'd never lay a hand on your heroine—and make sure she knows this!
...and always protect the innocent!
He cares about those who can not defend themselves, going out of his way to minimise casualties in battles, even charging into the thick of battle to save his soldiers. He'll also be willing to give up his life for the greater good.
He falls in love with your heroine at first sight...
From the moment he lays eyes on your heroine, he'll see her for the special, unique woman that she is. He'll be intrigued by her, and think she's the most beautiful, interesting person he's ever met. He may also be her "mate"—if he's anything like Cassian, Lucian or Rhys.
...and is totally devoted to her
He'll do anything for your heroine, including die for her.
He's sexually experienced
He's had a few women in his time so he knows how to please a woman sexually, but he's willing to swear off all others to pursue your heroine.
He cares deeply about her emotional wellbeing
He's in tune with the innermost thoughts and desires of your heroine; it's almost as if he can read her mind. He knows what she's thinking even before she says it, but wants her to say it just so he can hear her voice. He's aware of what she's feeling, even if she isn't aware the she feels that way yet. He knows what's best for her, but would never push her to do anything which she didn't want to.
Here is how to write a protagonist like Sarah J Maas.