The Mediator Series by Meg Cabot

Love You To Death
The first in the series starts off right at the beginning with Suze moving to California to be with her mum's new husband and his three sons. If that wasn't bad enough, her new bedroom already has a tenant; only problem is he's dead. 
Suze was always my favourite heroine; she's sassy and tough and smart, and although she might whine about her ability to see dead people, she has learned to take it all in stride and do what she can to help them. 
Her new Catholic school, founded in the 1700's, should be swarming with ghosts but Suze finds it empty. All due to the hard work of the only other mediator she's ever met, her new principal Father Dominic. I adore Father Dom, he is the complete opposite to Suze in his ghost-handling but they compliment each other.
If Suze didn't have enough to deal with, she is now the target of homicidal ghost who really doesn't want to be dead. But what can Suze do when Heather killed herself over a guy? A great start to the series and only sets it up for more amazing to follow.

High Stakes
There's a pretty new boy in Suze's life: Tad. What he lacks in conversational skills he more than makes up for in muscles. The only problem is his father thinks he is a vampire. Ghosts are one thing, but vampires? Nuh-uh. Suze also has a murder to solve, as well as a not-murder. She is woken by screaming - one way to get her attention - and the woman is adamant that "Red" didn't kill her. Suze goes on the hunt for this so-called Red and stumbles onto the body-filled path of Mr Beaumont. 
Despite having successfully dealt with Heather, the situation with the Beaumont's felt a lot more sinister and serious, maybe just because he was a grown man rather than a pissed off teenager. Either way, Suze gets into a few scraps but just manages to make it through alive. Her relationship with Tad might have been short-lived but her secret crush on Jesse is developing rather well.

Mean Spirits
Four teens have suffered a fatal accident and Suze has solve the mystery of what happened - she gets close to the lone survivor who was driving the other car. But some things don't quite add up and she has to go mediate some ghosts looking for revenge with the help of Father Dom and Jesse. Plus Suze's best friend from New York is visiting and Suze is struggling to keep her ghost-hunting a secret.
I really like this one, the ghost-hunting story is quite personal and leaks into Suze's normal life, which is both awkward and harmful. It's also quite a tough one when a teen lad is deciding that the popular kids don't deserve to live. 

Young Blood
Suze is working as a baby-sitter over the summer at the resort, where she meets the second mediator of her life. Problem is he's a whiny little brat whose been labelled as a crazy person. Suze straightens him out and gains a student, pity her teaching methods aren't exactly up to scratch! Plus his older brother Paul is a bit of a nuisance, a hot nuisance but still up to something that worries her.
And then there's Maria and her slave-runner, deadbeat husband Diego who fear repercussions for finding Jesse's body. Because it's always nice to be woken up by the ghost of the almost-boyfriend's ex-fiance holding a knife to your throat.
I'm a romantic, it doesn't matter that Jesse's dead, I think they have a great relationship and I want them to be together. This is the first that we see that might be a possibility. Until of course Jesse disappears and Suze is grief-stricken and so desperate to get him back that she mediates herself! And that's where two things happen: one, Jesse realises how important Suze is to him and two, we find out what the deal is with Paul and man is it a doozy.

Grave Doubts
Paul has decided to stick around California, for reasons he will not share but is definitely up to no good. And since they didn't leave Paul in the best of places, you know the corridor between heaven and hell, Suze is understandably hesitant to trust him. Paul and Jesse come to heads again. In fact, Paul must be some kind of stupid because he throws their "relationship" back in Jesse's face, because Paul is an egotistical jackass and thinks he's better than Jesse because he has a pulse, and Jesse throws down in the best way. Bad timing, as the house is full of guests, but it is the funniest and most dramatic fight scene with added awkward when it looks like Paul is being held down by thin air.
Sleepy has a new friend from college, pity he has a ghost following him. Suze then has to deal with a dead guy who thinks his brother should have died instead of him, as well as Paul antagonizing Jesse, and only has one last-resort option: use her newly-discovered shifter powers and travel to the dead realm. 
Things really heat up in the penultimate book, both between Suze and Jesse, and with Paul and his weird, super-mediator attitude. I love the development of Suze's relationship with Jesse, and in herself and her new place in Cali. She might only be sixteen but she's put up with a lot and yeah, still acts like a dramatic teenager but is tough-skinned and practical and I adore her attitude.

Heaven Sent
Suze finally has Jesse's heart but Paul is determined to break them up. They have a delicate understanding that Paul won't try and suck Jesse into the aether if Suze takes weekly mediator lessons, which is basically an excuse to try and get under her shirt. But Paul has a trick up his sleeve: he has found a way to travel through time to stop Jesse from dying. Reading this as an adult, I realised that Suze sounded like a stroppy kid having a tantrum, but she is also in love and love is very selfish. 
I don't want to ruin anything but the second half of book is that they do succeed in traveling through time. Suze clearly wasn't prepared for seeing Jesse alive, it completely knocked her back and she realised she couldn't stand in the way of him living a full life. But it's Suze and it doesn't exactly go well. Finally, I have to talk about the ending. My romantic heart bursts when I think about it, which I still think is the best way Cabot could have made a happy ending for them. 
I adore this series so much and I really enjoyed re-reading something that was so much a part of my teen years. A highly recommended series for fans of old-school teen novels, fantasy romance and a proper kick-ass heroine.