Sarah J Maas is the queen of writing young adult fiction. Here is how to write a protagonist like Maas.
She's a female
This is non-negotiable.
A rose by any other name...
No plain-jane names. Your protagonist must have a poetic, beautiful name.
A strong personality
She'll start off with a cold strangeness to her which masks her compassion and love for her friends, her family and strangers. When she cares for someone, she cares for them deeply. When she is happy, she is witty, charming and funny; when she is unhappy, she is withdrawn and can lash out at the people trying to help her. She is unwaveringly loyal, passionate and compassionate. She must have a very strong sense of justice. She must also have a dark side to her, being able to mask her emotions when needed, and she is able to kill people or manipulate those around her (but only for the greater good).
A dark past involving family trauma
Terrible events have happened in her past which have left scars — wounds which only a very special male can help heal. These scars will particularly revolve around family trauma, usually the death of a parent. She will have experienced poor living conditions while she was a child — going to bed hungry, cold and tired. She will have been mistreated by a father or father-like figure, and was shown no love by a sister or mother.
She is pretty, but also unique
She's beautiful, but in the way where her beauty is very unique. She's tall, with great hair. Close attention should be kept to her eyes — they should be an unusual colour and should be very expressive, showing the deep emotions which she feels.
She must undergo a transformation...
Your protagonist must transform radically. This transformation must be of her social position or nature, whether she turns from human to fae, slave to queen, or commoner to High Lady. She must also undergo a physical transformation, a makeover, which will include new dresses, fancy hairdos and perhaps some makeup.
...and become very powerful
Part of her character's arch will be discovering the amazing powers which she has, and then learning how to control these powers.
Find her a worthy mate, but there were almost others...
She must meet her male mate, and fall deeply in love with him. She'll meet other men before her mate, have a romance with them, but realise that they're not right for her. The romances which don't work out should be a love-hate relationship.