I know this is about Diabolik Lovers, but I just want to make something clear.
In general, do not do the following IN ANY FANDOM, unless your trying to make it cringe worthy.
If you are trying to write a good OC, that isn't a run of the mill boring everyday uninteresting character, don't do the following:
Have a name that resembles: Scarlett Silver, Skylar Storm, Ruby Roundhouse, you know, anything that sounds like a name out of a porno.
Don't give your OC powers that make no sense to the story. If you do give them powers give said powers limits and don't make your OC as strong as the main character, actually having them start out weak is always good.
Don't give your OC an edgy background or sad past. It's over used and frankly if you don't have a sad past, then don't do one. I've read so many stories with characters who, example, have an abusive father, who stabs them or something, and traumatizes them. Then they are brought to (Insert Story Here) and suddenly they don't have any form of trauma, it's as if they were NEVER in a bad situation in the first place.
Don't make all the characters fall in love with your OC. Especially the villain characters. If you must, I'll make a chapter explaining how, because dear god does that have so many land mines.
Don't spend an entire paragraph describing your OC, example: She had long red and purple stripped hair with blue eyes and wore combat boots with a skull t-shirt, she had a lip piercing and blue skirt, etc.
That manner of writing interrupts the story, then don't do it. I can assure you nobody actually reads that part, it's boring and unimportant. Don't give to many details about the character all at once. Mention the eyes and hair color throughout the story, from other character's point of views, and make sure it's not describing her like I just did. Here's a way to better explain how somebody looks:
Examples:
He watched her, interested, red hair blowing in the wind, as her blue eyes glared at him furiously as she adjusted blue skirt as it blew up in the wind. Or she toyed with the piercing on her lip nervously. Or she pulled her long skull skirt (It was her favorite) shirt over her pink tank top. Or she glanced down at her combat books and hoped the tied weren't undone.
Next, don't make your character related to one of the characters in the Fandom, it's hard NOT to make those corny and cheesy. I'll do a chapter on how NOT to make them crappy if you must.
Stop making character's act OOC because they're now friends with your character. This is why people call them Mary-Sue.