Morning Rising by Samantha Boyette is a young adult fantasy about love, abuse, drugs and fairies. It's the first book in the Guardian of Morning trilogy. It is mostly urban fantasy, spanning multiple worlds. I'm not sure what 'species' they actually are - elf might be closest, or it might just not matter.
Kara and Dylan are two miserable teenagers living in some city somewhere, in the care of neglectful single mothers (Dylan's mum is a drunk, Kara's has an abusive boyfriend. Abusive to Kara, that is). They meet by apparent chance (actually, by frustrated intervention from a third party, due to a long stream of missed meetings), feel an instant bond, and become each other's only real friend.
Kara is desperately in love with, and attracted to Dylan from the start, but Dylan is too far gone to accept that, instead pushing Kara to see how much she'll take, or ignoring her in favour of any boy (or girl, I think - it's mostly boys, but it's implied that Dylan doesn't really care) who takes the time to admire her, promise her momentary affection or offers her drugs. It doesn't help that Dylan is attractive and the instant center of attention wherever she goes. Dylan does occasionally open up enough to respond to Kara's unspoken desire, but she always frames it as something she's doing
for Kara, never actually admitting she feels the same way.
She does, of course, but Kara isn't sure of that for much of the story, and it means that there are two conflicts to be defeated, actually rescuing Dylan, and Dylan and Kara finally resolving their personal and romantic issues. The latter is made a
lot easier by the rest of the plot explaining or removing some of the barriers between them (such as Dylan's propensity for drugs, and Kara's underlying need to protect her).
Turns out that neither of them actually belong in the human world, but were placed there for Dylan's safety (and Kara went with her to protect and care for her). Both of them were terribly weakened, both magically and emotionally, by being in the human world, making them less able to cope with temptation, or stand up for themselves. Once they are returned to the other world(s), they regain strength of character and their magical abilities start to return.