The supernatural crime thriller series Death Note asks all kinds of tough questions about morality, justice, human ambition, madness, and much more. It all starts when high schooler Light Yagami gets his hands on the killer notebook and uses it to rid the world of crime, but becoming Kira warps his mind beyond recognition while the super-detective L pursues him with all his might. The story of Death Note is chronicled in the 37-episode anime series, but the universe expands to several live-action movies and TV specials, too, not to mention a recent one-shot manga chapter.

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Light and L dominate the screen, but often, Death Note's most tragic scenes involve the innocent bystanders who get caught in the crossfire, such as Light's little sister, Sayu Yagami. She doesn't appear much on screen, but she is family, and her life takes a turn for the worse when her brother becomes history's greatest serial killer. What are ten things we should know about her?

10 She Has A Sense Of Humor

Sayu and Matsuda from Death Note

Sayu Yagami is hardly a goofball, but then again, she knows how to do a little good-natured teasing to the people she likes. Earlier on, when Light started "dating" Misa, Sayu joked that it would cost Light 5,000 yen (around $50) to keep the secret safe.

Later, as pictured, Sayu teased Matsuda, saying that she would have considered dating him if he were closer to her age. But she was just joking, and Matsuda totally fell for it. She's a crafty one, all right.

9 She Likes Trendy Things

Sayu, for the most part, likes whatever's popular, and she wants to be part of the "it" crowd. For example, she's really into pop idol music videos, and Hideki Ryuga, the latest heartthrob, has captured her attention 100%.

Not only that, but she keeps up with fashion magazines too, and when Misa visited the Yagami house, Sayu knew exactly who she was, even though Misa's major stardom was still in the future. And Sayu was thrilled that her brother was seeing such a beautiful model.

8 She Is Nosy About Light's Love Life

Light and Misa staring off screen in Death Note.

Light and Misa are quite the couple, aren't they? At least on the outside, they seem pretty normal, and Sayu likes to keep tabs on her brother's love life. She was a little apprehensive at first, and thought that Misa was an iffy person for Light to date.

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Sayu (and Mrs. Yagami, for that matter) soon warmed up to her though, and as mentioned earlier, Sayu was quick to tease Light about Misa. Not to mention when (in part 2) Sayu strongly suggested that Light and Misa hurry up and marry already. Imagine having Misa as an in-law!

7 Ambiguous Recovery

Sayu was traumatized when Mello's thugs abducted her and flew her over to the U.S., and they held her captive in an underground facility in the Southwest until Light and Mr. Yagami hurried over to trade a spare Death Note for her very life.

This was very hard on Soichiro's younger child, and we see her being pushed around in a wheelchair. But in the final scene where she appears, Sayu is taking part in a public coming-of-age ceremony, and she looks calm and happy. Is she? We can't say for sure, and the writers wanted it that way. There's no telling if Sayu will ever be the same again.

6 She Idolizes Light

Here's another positive thing about Sayu: she is an adoring little sister. Often, siblings clash in anime, or they're even trying to kill each other. Just look at Sasuke and Itachi, for example. But not here: Sayu absolutely adores her big brother, and doesn't hide that fact, either.

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Just to lay it on thick, Sayu is one of several characters who openly praises and comments on Light's brilliant mind, and she supports his plan to join the NPA to work with his father. Amusingly, Sayu once thought to herself that Light is "starting to become a real teenager" while making a little smile. We'll assume that's a compliment!

5 She Represents Light's Lost Innocence

sayu yagami happy

This might not have been confirmed by Death Note's creators, but we feel confident in this interpretation of Sayu's character. After all, why else give Light a little sister who adores him and their father? Yes, there is the kidnapping scene, but anyone important to the NPA could have been used for that plot sequence.

We are certain that Light has a little sister so that she can contrast with Light's descent into darkness. The both of them began the series as the adoring children of a tough cop, but Sayu remained innocent while Light became a monster, just as a measuring stick to show how far Light has fallen. And Sayu doesn't even realize it.

4 She Is Protective

soichiro yagami in the death note anime

Here is Soichiro Yagami, Light and Sayu's father. Early in the story, the entire NPA is struggling to deal with the bizarre and deadly Kira case, and detectives were quitting due to both the difficulty and danger of this case.

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Many cops said that they wouldn't face death to capture Kira, since they had families. Soichiro has his own family, and both his wife and daughter urged him to quit the case. It hurt them to even imagine Soichiro dying to Kira, but all the same, Soichiro persisted.

3 She Loves Snacks

This is a visual gag more than anything, but it does tie into her theme of representing the innocence that Light lost on his quest to rid the world of crime. Sayu is a bit of a homebody, and she likes to lounge around, as opposed to Light, who keeps running around both for school and his grisly Kira career.

Sayu likes to get bags of potato chips and other snacks, and what's funny about it is that she once teased Light, telling him that he would get fat if he snacked on potato chips while studying. But we all know what Light was really using that potato chip bag for!

2 She Fit The Kira Profile

Does this look like the face of Kira? We don't think so, and it's ludicrous to even imagine her using the Death Note at all, let alone launching a campaign against criminals en masse. How did she end up fitting Kira's profile?

L correctly deduced that Kira is a young adult who is fairly well off, and a student who has access to police databases. This certainly fights Light's profile, but Sayu matches it too, and Light was outraged when L casually said this right in front of a hospitalized Soichiro. What's odd is that Light himself described someone just like Sayu being Kira, but Soichiro firmly (and correctly) said that his daughter could never be Kira.

1 She Can't Stand Kira

Poor Sayu suffered horribly when Mello's thugs captured her as part of his plans to defeat Kira and become the new Death Note master, but even before then, Sayu was decidedly anti-Kira. She just wanted Kira to go away and never come back.

Not only would an innocent and upstanding girl like Sayu oppose someone like Kira on principle, but her beloved father is overworked and in grave danger because of the Kira case (he was away from the family for the holidays due to the case). She openly said "I hate Kira" because of this, and she just wanted her father safe and at home with his family. If only it could be so.

NEXT: Death Note: 10 Weird Rules Light Yagami Has to Follow