To be fair, being a mary sue doesn't mean you're incapable of being afraid or being incompetent. Bella Swan is one of the best examples of a Mary Sue we've had in the last decade yet she spends a lot of her time being either afraid or confused.
Not that Shuri is a mary sue of course, just saying there are other reasons why she's not a mary sue other than simply being afraid.
I'm not super familiar with the Twilight movies. I saw them once but forgot almost everything. How is Bella Swan a "mary sue"?
Off the top of my head:
1. Bella is described as clumsy, anti-social and not-very-attractive. Yet all the guys in her new school seem to be crushing on her including the two hottest hunks, one of whom is a vampire who has never been interested in any other girl before and the other a werewolf with muscles like a greek god who also is not interested in any other girls except her.
2. Edward is apparently attracted to Bella because she "smells" different and it's like an aphrodisiac for him. This "smell" is never explained further. Basically it boils down to Edward being head over heels attracted to Bella because... well, because they say so. He's also attracted to her because he can't read her mind. He's a telepath and she's the first person he met that he couldn't read, and apparently this makes her irresistable for him. I don't believe it's everexplained how she has this "blank" power.
3. The wolfboy (forgot his name) is in love with Bella because he just instinctively does. He "imprints" on her, at least that's what they called it. More on this later. Point is he doesn't have a reason for liking her, he just does.
4. Vampires are ravenous for human blood but the Cullens managed a "vegan diet" that consisted of feeding on animals but they only achieved this after years and years of strict discipline. When Bella got turned into a vampire, she curbed her appetite for human blood immediately and without much effort. Speaking of which...
5. New born vampires are the hungriest vampires of all, usually going on a crazy feeding frenzy for a few months after they're born, almost mindless zombies, which makes them very dangerous and very hard to control. Bella was completely lucid after being made a vampire and was able to completely control her appetite for human blood.
6. New born vampires are also physically the strongest vampires but that's the limit of their powers. When vampires get older, they get weaker physically but they develop other esoteric powers (like telepathy, precognition, etc.). When Bella got turned, not only was she physically the strongest vampire out of all of them (defeating Emmett in an arm wrestling match who was considered the strongest Cullen) but already had ridiculously strong esoteric powers as well (she could create invisible shields).
7. Back to her love triangle. She eventually gives birth to a semi-vampire child and it turns out that she passes on to her child her "aura" that had made wolfboy instinctively fall in love with her. So now wolfboy is no longer attracted to her (removing her need to choose between him and Edward) but is instead head over heels in love with her baby daughter. But if you think the idea of a 17-19 year old teenage boy falling in love with an infant is creepy, don't worry because...
8. Her daughter has accelerated aging. And within the span of a few weeks (or maybe months, can't remember) she's already both physically and mentally the age of girl in her late teens. Oh and she's also head over heels in love with wolfboy. But won't that mean that she'll easily out-age him? Well no, because apparently her speed aging is only until she's in her teens and then it slows down and goes back to normal once she hits that age. So how does this go back to Bella being a Mary Sue? Well, because...
9. She now doesn't need to choose between Edward and wolfboy, because she gets to marry Edward and still maintain wolfboy as her bestfriend without it weirding out anybody because wolfboy is now set to marry her daughter instead. So they're one big happy family. Speaking of which:
10. Some of the Cullens didn't like Bella at first. Hated her in fact. Yet they eventually all love her and treat her as one of them even before she became a vamp. And she managed to do this even though she's anti-social, kinda rude, didn't make any effort to win the Cullens over, and just plain not likeable at all. She even admits that she's not likeable.
11. Did I mention that all the other evil vampires are intoxicated with Bella as well? That's what makes Edward's life so difficult, because all the other vampires just have to have Bella and he's kept crazy busy trying to protect her from them.
12. Did I mention that vampires in this universe don't really have any weaknesses at all other than their dependence on human blood, which somehow Bella doesn't suffer from when she became a vampire? I guess they're also kinda susceptible to fire, but her invisible shields easily take care of that.
13. This one is from the books (as I don't remember if they showed it in the movies) but IIRC, the process of changing a human to a vampire is extremely painful, long and difficult. Yet Bella was able to use her "blank mind" powers to suppress the pain and so her vampire transition went pretty smoothly and without much difficulty.
14. Oh and apparently she also became extremely beautiful when she became a vampire. In the books anyway, she described not recognizing the goddess that she saw in the mirror (yes, twivamps have reflections).
Anyway, I could probably keep going (and that's only a list off the top of my head) but you probably get the picture. Basically she spends most of the movies being this unlikeable, scared, mostly confused clutz yet she ends up getting everything she wanted and the best of all worlds handed to her in a silver platter.