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I have put three characters to this test. One is an OC in the Hogwarts-verse. One is the main character in my comic. One is me.

Sadako (title character of The Muggle Born Slytherin) Shimisu

Is the character named after you (this includes your first, middle, and last names, and any nicknames or online names you use)?
I almost missed this one. I do cosplay Sadako. I probably wouldn't have thought of the name if i didn't.

Is the character's name just generally unusual in relation to the time or place where she/he lives or grew up?
Yeah. Although it's mentioned she's named after a relative and that her father's family is japanese, most english girls are not named Sadako Shimisu.

Is the character's name foreign, even though the character isn't?
     Is it an Asian name? The fact she's Asian doesn't help me at all, does it? Shit.

Does the character have a name or nickname that corresponds to a mythological/literary/historical/etc. figure (e.g. Venus, Faustus, Bonaparte)?
Sadako is the name of the ghost in Ringu. Shimisu was the surname of someone working on Ju-On.

Are one or more other characters attracted to her/him?
She does have a love interest. Just one. The fact he is also an OC doesn't help either, does it?

Does the character have unusual or exceptional hair, or does her/his hair get a disproportionate amount of description compared to that of the other characters?
Well, everything has a pretty absurd amount of visual detail, so it isn't described more than the other characters. But there are several references to it being unusually static-y.
"So I'm a witch." Sadako said slowly. "Is that why my hair is all... you know? Magic makes it stand on end?"
"I'm afraid nothing can explain why your hair does that." Ms. Burton said quickly then covered her mouth, as if she was ashamed of saying it.

Is the character of above average intelligence?
Only about as much as I am.

Does the character have an angsty childhood, or an angsty past?
She didn't have many friends in school, and she saw someone die when she was very young.

Does the character feel guilty about something terrible that happened in the past? Even despite overwhelming evidence that it wasn't her/his fault?
No. Despite mounting evidence that it might well have been her fault.

Does the character have a faithful pet or animal familiar?
I wouldn't call Whisper faithful, but what do you want, she's a cat. (There should be a checkbox for does the pet get a sideplot. Over the seven years at Hogwarts, Whisper transitions from housecat to familiar. That's got to lose Sadako a few points.)

Has the character ever fairly lost in any kind of duel, fight, or competition against someone of equal or lesser ability, where the winner was not the character's rival?
It was only mentioned in passing, but yes. It happens when you're in a dueling club. And there was the time she picked on someone far above her ability and got her ass handed to her. That was funny.

Does the character have an accent relative to her/his country of residence?
Should a British girl not have a British accent?

Does the character have a major quirk or habit in common with you (e.g. drinks the same brand of soda as you, talks to pets the same way you do, etc.)?
I'm going to count long dark hair and being surly.

Does the character have a significant personality flaw (e.g. she/he is a spoiled brat, is horribly judgmental or biased, is irrationally violent, is naive and easily manipulated, etc.)?
I think being surly and nasty-minded counts as a flaw.

Does this flaw get the character in serious trouble (e.g. she/he gets fired, gets killed, gets someone else hurt, etc.)?
Well, serious for school. If someone died because she mouthed off that wouldn't be terribly realistic. She has gotten detention, not enough for it to be her second home, but she has.

Sadako's Sue Score! 30
Borderline-Sue. Your character is cutting it close, and you may want to work on the details a bit, but you're well on your way to having a lovely original character. Good work.

She probably would have passed if she wasn't Japanese.

Kunni
Is the character named after you (this includes your first, middle, and last names, and any nicknames or online names you use)? Her full name is Catherine-Elizabeth Emily Alice Rose. It doesn't come up much. ("I think my parents wrote fanfiction, too.")

Does the character have more than one first, middle, or last name, without reason? I don't suppose "It's funny" is a reason?

Does the character have a nickname or pseudonym?
Wouldn't you?

Is the character highly attractive only through her/his concerted effort?
Look out for the "Kunni without makeup" story arc.

Does the character have unusual or exceptional hair, or does her/his hair get a disproportionate amount of description compared to that of the other characters?
Everyone in the comic does.

Is the character a fashion plate (this applies to any style of dress, from preppie to goth to Western to Elvish, and so on)?
Again...

Is the character of above average intelligence?
Only so much as I am.

Does the character use a katana or any other Asian weapon? Despite being of non-Asian heritage?
I think that's pretty much all of Kunni.

Does the character have a major quirk or habit in common with you (e.g. drinks the same brand of soda as you, talks to pets the same way you do, etc.)?

Does the character have a significant personality flaw?
Irrationally prone to ranting and blathering. Unaware of reality.

 Is the character, or was she/he ever...
A ninja?
A starving artist?
A freelance writer?
A gymnast?
She's just going to fail...

Does the character possesses the following skills, with or without the aid of charms, spells, etc.
Skill in martial arts or hand-to-hand combat
Sleight of hand

Does the character share more than one of your hobbies or interests?
Well, she was based off of me what do you expe- I'm not helping this at all...

Does the character have the same taste in music as you have?
            In books? .... *shifty eyes*
            In movies?

Does the character have the same religious or spiritual beliefs as you?
Well, I'm pagan and she's wiccan, but that's close enough for most people.

Do all of the important characters end up liking/respecting/fearing her/him?
Well, the plot centers around her friends... crap.

Does the character fall in (reciprocated) love with, or have sex with, a character you would like to fall in love with or have sex with?
Well, I guess Shino's a nice guy...

Do you draw the character, or ask other people to draw her/him for you, more often than any of your other characters?
Actually, I don't. She's the main character of my comic, and I don't. *shame*

Did you base the character on your favorite previously existing character or person?
Me, okay?

Is the character a mouthpiece for your own beliefs or views?
Only about Myspace and Twilight.

Kunni's Sue score! 80!
71 points or more: Irredeemable-Sue. You're going to have to start over, my friend. I know you want to keep writing, but no. Just no.

Me.
Does the character have more than one first, middle, or last name, without reason?
Does the character have a nickname or pseudonym?
Is the character highly attractive only through her/his concerted effort?
Are one or more other characters attracted to her/him?
Even though they are involved with someone else? [ughuhuhuh.]
Is the character a fashion plate?
Does the character dress in a manner you find particularly attractive, sexy, or cool?
Even though it's illogical for the character to dress this way (for monetary reasons, or because it interferes with her/his job)?
Is the character of above average intelligence?
Does the character have an angsty childhood, or an angsty past?
And does she/he angst about it during the story? (Angsting does not necessarily include simply relating events to an interested third party.)
Does the character have angst in the present?
Did the character ever suffer physical or emotional abuse?
Did the character ever suffer sexual abuse?
Does the character feel guilty about something terrible that happened in the past?
Even despite overwhelming evidence that it wasn't her/his fault?
Does the character have a faithful pet or animal familiar?
Does the character use a katana or any other Asian weapon?
Despite being of non-Asian heritage?
Does the character "just know things"? [to quote the forums: To save time, it is better to assume that Kitty knows everything]
Does the character have a physical handicap that does not hinder her/him significantly?
Is the character significantly mentally handicapped?
Do animals or children instinctively like the character?
Does the character have a significant personality flaw?
If the character does not die, does this flaw persist beyond the end of the story?
Does this flaw get the character in serious trouble?
Does the character have a physical "flaw" that does not actually detract from her/his beauty? Uneven face. No one seems to notice it.
 Is the character, or was she/he ever...
....A starving artist?
....A freelance writer?
Does the character share more than one of your hobbies or interests? [I have no idea how to answer that]
Do you take any negative feedback about the character as a personal affront? [Uh, I'm taking this for me. Wouldn't it be a personal affront?]
Is the character a mouthpiece for your own beliefs or views?
Do you have a character journal for her/him?
My Sue score: 50
36-55 points: Mary-Sue. Your character needs some work in order to be believable. But despair not; you should still be able to salvage her with a little effort. Don't give up.