The Melancholy Symptoms of Unrequited Love

Unrequited: not requited :
not reciprocated or returned in kind
<unrequited love>

Alyxx was very much in love with her best friend of 16 years; she has been since as long as she could remember.

Shaun was in love with his girlfriend of four years and only saw Alyxx as his best friend since pre school.

He was her unrequited lover.