Happy Birthday to a special young woman and a kindred spirit, on your 23rd birthday.
 

Trying to get this right. She is a Mexican-America born in the U.S. Her parents immigrated to the U.S. from central Mexico

"To Xxxx, a very special young woman and a kindred spirit, on your 23rd birthday. Thank you for being a friend. -- Xxxx"

"Happy Birthday sweetie! (or dear)!"

Our relationship is fairly close but we are not romantically involved at this point and may never be. I do feel some affection for her and she is quite a bit younger than I am

Try your own attempt first without an online translator and we'll be able to help you correct. :)

Okay, I tried
 

Trying to get this right. She is a Mexican-America born in the U.S. Her parents immigrated to the U.S. from central Mexico. I am a white male.

"To Xxxx, a very special young woman and a kindred spirit, on your 23rd birthday. Thank you for being a friend. -- Xxxx"

"Happy Birthday sweetie! (or dear)!"

Our relationship is fairly close but we are not romantically involved at this point and may never be. I do feel some affection for her and she is quite a bit younger than I am. Is "querida" or "cariño" appropriate for this situation.

"A Xxxx, una mujer joven especial y una alma gemela, en tus 23 cumpleaños. Gracias por ser una amiga. -- Xxxx"

"¡Muy feliz cumpleaños querida!" (o cariño)

Para ti, ABC, una joven muy especial, espíritu afín al mío, en tu vigesimotercer cumpleaños ¡Gracias por ser mi amiga!

¡Feliz cumpleaños, querida!