okay, so it's pretty obvious that some people are prone to writing characters incorrectly, i'm simply going to go quick and remind all of you of who you're writing, if you're looking for something more detailed, either go on the vampire diaries wikia page or check out the character analysis that crescent00moon  has done on her book HOW TO WRITE A FANFIC FOR THE TVD UNIVERSE, her's are really well done and i would most likely take too long in explaining them

Elena

-  is a generally caring and nice and decent person, she isn't portrayed as paranoid or crazy or even mean during the first season, so don't push her under the bus just to make your OC look good. In season two is when she gets a bit more worried due to all the death that has taken place, not to mention Katherine showing up and then being served on a silver platter for Klaus to sacrifice. In season three, she's indecisive, she's starting to care for Damon as she would Stefan, and after Stefan goes all no-humanity Ripper, well, she clings to the other brother. When she gets to season four, well, yeah, you can start hating her then.

Stefan's

-  character can either be thoughtful and nice or an asshole, that's the majority of crescent00moon's analysis of him anyway, I have my own thoughts, however. Stefan is prone to shutting off his humanity, and it's used as a cop-out, he can't deal with the pain and guilt of what he's done so he decides not to feel any of it, it's just pausing the guilt because sooner or later he will have to flip that switch on. Please refer to my 'a vampire's humanity switch' chapter for more.

Damon

- can be an interesting character to work with. In season one, he has a goal, and that's Katherine, in season two, he's a bit more wayward, and by season three, he's full-on in love with Elena. In season four he's basically on a rollercoaster because hey, he finally got the girl, but then it turns out she's sired to him, and it creates a whole big thing, you know this, you've seen it, you just have to pay attention to the certain character you're trying to portray. By the last few seasons, he's not the good guy, he's just willing to do whatever it takes to save his loved ones, and sometimes that means not doing a good guy thing. he's willing to do the dreaded thing that everyone is afraid of doing because he can see how it'll end badly, he already has enough blood on his hands, what's a little more?
Please check crescent00moon's analysis for further details

Bonnie

-  wants nothing to do with vampires very early on, she's a witch, it's what they were taught. Around season two you can see her slowly go over to join their side because they have a common enemy: Klaus. Why? 'Cuz he wants to kill Elena. Around season three she's already pretty invested and by season four she gets mixed into expression, Jeremy dies, Silas is in her head and it just ain't pretty. In season five she's obviously dead and then suffering as the anchor to the other side, later on in season six develops depression and slight PTS from being alone in the 1996 Prison World when Kai escapes.

What you can gather from these seasons and later on is that Bonnie is generally the one that comes up with the solve for the group and also the one that suffers the most (because of solving it usually) And now I have to stop myself because I will go on a rant on how she deserved so much better, but overall jut remember: Bonnie is the best.

Caroline

- wants to grow up, she wants to graduate and do bigger things, she's better off being a vampire, she grew into herself by being this way and is a strong and great character. In season one she is insecure and a control freak and very perky, by season two and onwards she becomes a better person. Her relationship with Klaus in seasons three and four is complicated and if Klaus is the main love interest for your OC, you cannot just write off Caroline and place your character in her place. Klaus actually brings a different perspective to Caroline, and she's the reason he tries to actually be better or at least less cruel than he usually is.

Tyler

- is in desperate need of a pack, that's what I would say, in season one he was a jerk, i'll admit, but by season two we can really see him evolve, the werewolf curse is the worst thing he could've imagined and he was willing to do anything to get rid of it. In season three we were off to a good start but then he got turned into a hybrid and become a Klaus minion, which caused one of the few diverse actors on the show less screen time, whoo hoo. Honestly, this kid deserved better. I remember in early season 4, he sacrificed himself to Connor at the church so that in the chaos, his friends could escape. I really liked that Tyler. I was honestly hoping for him and the rest of the gang to sort of develop into this family, oh well. I'll get to that in a whole other chapter where I compare TVD and True Blood (spoiler alert: TVD is way better)

Marcel

-  is a character that has undergone a seriously abusive childhood, i mean he was a slave, because of that childhood he is greatly against having children mistreated in any way shape or form, Marcel actually has a rule that he stands by no matter what: you do not mess with kids. In season 1 he's a vampire king of a whole city and one of his rules is to not abuse kids, sure he's also a lot of other things, but I think that was an important reminder. Overall, he cares about his city, but never forget that he's only willing to help the vampires. A reminder that he got the werewolves cursed to the Bayou and the witches under his heel during those early episodes. So he's obviously very resourceful and intelligent, a reminder that he won that weird contest the Strix had against Elijah and Aya back in season three, he went on to say that it was a battle of intellect and wit.
Marcel was who the prophecy was about in that season as well. He was both foe, friend and family to different members of the Mikaelson family.
Marcel by season four is the self-proclaimed king once more but seriously blind to how much the city actually suffers because of his reign. He also prides himself on not being a Mikaelson during this season. He's considered as an older brother to Hope later on.
Also, Marcel is Davina's adoptive father, so whoever the fuck wrote that story where he's having a sexual relationship with her (a SIXTEEN-YEAR-OLD) and stringing her along for her power, HOW DARE YOU, just do one thing:
DELETE DELETE DELETE.

If you want to check out the Mikaelson mindset

in greater detail, please check out Kaci  and her stories Blue & Black Magic, they're both from Henrik Mikaelson's perspective and his thoughts offer a really well-rounded analysis into each of the Mikaelsons. Kaci  does a fantastic job on breaking down these characters analysis and gives us a character that would fit perfectly well into the dysfunctional mess that is the Mikaelson family.

but for a quick overview of them:

Freya

has longed for her family for over a thousand years, she's willing to do anything to protect them, even from each other.

Finn

doesn't want to be a vampire, never has, he thinks of his family and himself as monsters. Though he holds a sweet spot for his mother and sister Freya.

Elijah

would do anything for his family. By season two of The Originals we see how messed up Elijah's psyche really is, he's been bottling a lot of that monstrous side away from himself. Elijah has prided himself on being the noble brother, always true to his word, and though he is, this does not make him the good brother. None of them are.

Klaus

is paranoid but intelligent, he's possibly the most intelligent out of all his siblings. He's a master strategist that is never afraid to get his hands dirty. The one thing that he would give up everything for is his daughter, Hope. But if you're going into his psyche when he's first introduced or from a different time period then it's a whole other story. The one thing that Klaus wants is to break his curse and make more of his kind, he feels like a freak for being his only species and has major abandonment issues. So, he's willing to go to any length to break the curse in season two, and later on willing to do anything for his hybrids to transition in season three. Klaus also has undergone an abusive childhood, this makes him sympathetic to children and he would never abuse a child of his. He would also never force a woman (or man, or shim or whoever) to be with him sexually, so whoever wrote that story: DELETE DELETE DELETE and go think about what you've done.

Kol

misses his use of magic and spent centuries teaching other witches different types of magic while trying to figure out a way for himself to practice it. Soon enough it becomes more of a ploy to take down Klaus with a gold dagger, long story. Kol is pretty destructive is season three and four of vampire diaries, but honestly has a big change in personality thanks to Davina where he learns to care about someone more than himself.

Rebekah

has always wanted one thing: to be human. She wants to grow old with a big loving family surrounding her. That's always been her main goal. Rebekah is always known to be the siblings favorite, and apart from Freya, their father's favorite too. She also happens to be the one Original Vampire that has a sire line intact, (since we don't see her take the cure or hear about it any time soon, we have no idea how it turned out, so for now, she has the only intact sireline). She also wants freedom apart from human life, but that doesn't mean she isn't loyal, she's also quick to love and has more than enough experience with dead lovers.

For Legacies characters like Hope, Lizzie, and Josie:

please check out crescent00moon 's character analysis on them, though please consider the differences between Hope in season five of the Originals and season one of Legacies.

Read more about those differences in the vampire diaries wikia page about Hope