This Oc belongs to @CougarheartWarrior

Name: Shadowstar
Just gonna say now, both parts of this name are kind cliche... and it's a canon name so :b

Past names: Shadowkit, Shadow, Shadowpaw, Shadowstar
Uh, warrior name? Do we have a 10 moon old leader?

Name reason: pelt blends into dark shadows
Ok, makes sense but kind of basic

Appearance: skinny black she-cat with sleek fur, gray paws, muzzle, and tail tip. Right ear is torn, and a long scar across her muzzle. She has bright amber eyes that look almost eerie.
Her description is fine

Gender: Female/she-cat
As I assumed

Age: 27 moons, I guess
Waaaaay too young for leader. I consider adding 10 more moons or so before becoming deputy/leader.

Personality: very protective over her friends and loved ones, sometimes rude and snappy, clever and sly. She is very brave, her only fear is being defeated. Easily annoyed by others, and eager to learn, ambitious
Hmm... something about her personality seems cliche for some reason... I think it's the "only fear" thing. You can't be telling me she isn't scared of ANYTHING but defeat?

I would consider adding more traits though

Clan: Was PineClan, then a loner, then PineClan again.
Uh, okay. I'm interested to see how that plays out
PineClan lives in a big pine forest (obviously) and usually eat squirrels and crows, sometimes frogs. They have the same rules as other Clans.
Sounds like a solid clan.

Rank: Kit, Apprentice, Leader. (yes I know she wasn't a warrior or deputy before she became leader but I'll explain that later)
Better make it a good explanation! 🤨

Kin:
Father is Snakefang, a tan tom with darker stripes and large ears, and his two front fangs are unusually long and stick out of his mouth. Dark, pine-green eyes.
His description is pretty good XD

Mother is Breezewhisper, a white she-cat with dark gray dapples and a long, fluffy tail. She has dark amber eyes.
The appearance is a bit basic, but overall is fine

Siblings: Owlscreech, a brown tom with large ears, a dark brown stripe along his back, and bright green eyes.
Again, pretty simple but I works with the parents

Blackpelt, a jet-black tom with a white underbelly and long, tangled fur. He has a scar across his shoulder and yellow eyes.
I think the name is a bit boring and too similar to Shadowstar's, no offense. Maybe name his something to go with his white underbelly or yellow eyes.

Crush: first crush was Dapple, a fluffy pale light brown tom with dark brown dapples and yellow eyes.
A rouge? Makes sense she was also one for a bit. And I like his appearance

Then, she had a crush on Foxbite, a dark ginger tabby tom with pine-green eyes, and covered in battle scars. His paws are a dark brown color.
Reminds me of Rownclaw 😝

Mate: Foxbite, a dark ginger tabby tom with pine-green eyes, and covered in battle scars. His paws are a dark brown color.
Yup

Kits: Shadowstar had no kits, because she was barren.
Well, she's also pretty young so I find it odd they confirmed her as barren already. Eh, I guess she is almost 30, but I just figured she'd be busier than trying to have kits. Also, could they not adopt?

Mentor:  Pinetail, a brown tabby she-cat with blue eyes. 
Very simple but I can see her clearly. Try adding a little more detail though

But after Pinetail died, her mentor was Cardinalwing, a bright ginger tom with white paws and pale, light green eyes.
Hey! I really like him :3

Apprentice: none
K

Friends:  Oof, she doesn't really have any friends cause she was bullied, her only friends was her littermates: Owlscreech and Blackpelt.
Okay, I guess that makes sense. I wonder why she was bullied though...

Backstory:  Shadowkit was born into PineClan but PineClan's medicine cat, Juniperleaf, received several dark prophecies that tell him that Shadowkit is dangerous, and she will destroy PineClan.
First off, I wanna say I hate the whole "kit will destroy clan in the future" thing that some backstories have. It just seems unrealistic to me that StarClan would basically tell the Clans to kick out a child because of something that might happen. A lot of times the prophecy is self-fulfilled (their drastic actions are what cause the prophecy to happen) or misinterpreted.
Juniperleaf doesn't want to take any risks and let Shadowkit stay in the Clan, so he convinces her mother, Breezewhisper, to get rid of her.
That's cold AF and I'm now questioning his ability as a medicine cat and Breezewhipser's ability as a mother
Breezewhisper didn't want to abandon Shadowkit, she loved her daughter to much.  But after Breezewhisper has horrifying dreams of Shadowkit destroying and murdering her Clanmates, Breezewhisper agrees that it is best for PineClan if Shadowkit is gone.
B r u h. Really? StarClan is gonna send this mother horrific dreams of her Kit? Maybe she should have left WITH her to try to prevent Shadowkit from destroying the clan. Or, you know, not punish a Kit for something they MIGHT do
In the middle of the night, Breezewhisper takes Shadowkit out for an 'adventure,' traveling way out of PineClan territory.
Why not in the day...? Does no one else in the clan know what the prophecy is?
Breezewhisper builds a nest for her and Shadowkit to sleep in, but as soon as Shadowkit falls asleep, Breezewhisper leaves her alone and travels back home.
Again, cold AF... literally 😂😂Breezewhisper claims that a fox killed her daughter, keeping the dark prophecy a secret.
Ok, so the clan doesn't know. I'm surprised she was able to go far enough away in one night that Shadowkit couldn't simply follow her scent trail back.
Meanwhile, Shadowkit wakes up to see her mother gone and no where in sight. She is completely unaware that Breezewhisper abandoned her, and Shadowkit thinks that she just went hunting, and that her mother will be back soon. 
Aw, that's so sad 🥺
Shadowkit decided that she will wait for Breezewhisper to come back, but the little kit gets easily distracted and chases a butterfly.
I feel better now 😌
She chases the butterfly far away until she realizes that she's lost, and she runs into two rogues, Dapple (tan tom with brown dapples, yellow eyes) and Night (large, muscular, black tom with white muzzle and paws, deep blue eyes.)
Kk, I know Dapple from earlier
Night told Shadowkit that they will help find her mother when she says she's lost. Of course, Night was lying, he hated Clan cats.
Interesting. Is there a specific reason he hates them?
Instead, he decided he would kidnap Shadowkit and turn her into a rogue, for his rogue group.
As any reasonable rogue would do
Night and Dapple bring Shadowkit to he rogue group camp, and let her stay there for the rest of the day.
Only the day? I thought they wanted her to be part of crew
It sounds wasn't until tomorrow
Sounds?
that Shadowkit realized that she was still lost, and without her mother.
Uh, yeah? It's not like they went anywhere
Night had kept her busy and entertained on purpose.
No shite.
Shadowkit thanks Night for letting her stay and feeding her, but tells him that she needs to find her mother and her Clan.
Wow, suddenly she's a big girl. Exactly how old is she?
However, Night thinks of another lie pretty quickly, claiming that he saw her mother get killed by foxes.
He didn't say this earlier because...? (I mean to be more believable)
Shadowkit doesn't believe him, and demands Night to take her to her mother's body if she was dead.
Damn, she seems a little too smart suddenly
Night says that the foxes took her body away, and because Shadowkit is a three moon old, foolish kit, she believes him.
Ok, so she's three Moons, but I still think the believes him a little too easy. Like, why doesn't she ask why he didnt try to help her mother? Or what her mother looked like? She seemed pretty smart a second ago.
Shadowkit wants to go back to her Clan, but Night doesn't let her leave. Night also changed her name to Shadow, and Shadow adjusts to rogue life. 
Uh, what about her not wanting to stay? Did she just give up as soon as he changed her name?
She was trained to hunt and fight, and spent a lot of time with Dapple, forming a crush on the tom that is also Night's son.
So, he was kind of like her Rogue mentor? I get why she would crush on him though
Shadow easily almost forgets about PineClan, but she does have some flashbacks of her and her littermates playing in the nursery.
Seems to make sense
One day, when Shadow was a lot older (probably old enough to become a warrior if she was in a clan),  she was chasing a rabbit and wandered off to her former Clan's borders.
Interesting she would go that far. I wanna know if she's kind of pushed that far before, but never crossed, or if the rogues told her past their territory was dangerous or something
She recognized the scent, but couldn't quite remember what it was. Curiously, she followed the PineClan scent to figure out what it was, wandering closer to the Clan camp.
Patrols=Non-existent
She soon found PineClan camp, and immediately her memories of her former clan came back to her.  Shadow went back to the rogue group and told Night she was leaving and going back to her clan. Night couldn't prevent her from going now; she wasn't a kit anymore, and she probably wouldn't believe any of his lies.
Alright, nice
Dapple was sad to see her go, but refused to go to PineClan with her.
Makes sense, he's only known the rogues his whole life
So, Shadow finally found her Clan, and she waited by the border for a patrol to come by.
Wait, I thought she found the camp. Did she somehow sneak a look at it without someone noticing?
After some warriors found her and escorted her to camp, Shadow asked Whitestar, the current leader, (pure white she-cat with long, silky fur and green eyes) if she could join PineClan. 
Oooh, I like the way she looks
Whitestar said yes, she was always glad to welcome new cats into the clan.  However, Whitestar didn't recognize Shadow as Shadowkit, so she refused to believe her when Shadow told the leader who she was.
Seems like denial
Shadow became Shadowpaw, even though she already knew how to hunt and fight; she still needed to learn the ways of the warrior code.
Ok, but she probably wouldn't have to be an apprentice for more than 3 moons.
Sadly, Shadowpaw was bullied and teased by the other apprentices, for being a rogue.
Uh, but she's like old enough to be a Warrior. They have some nerve
A few days later, when Shadowpaw was sent to clean the elders' den, she was surprised to see a familiar face.  She recognized the scent immediately, it was Breezewhisper.
JESUS. How old was she when she had her kits?? Shadow isn't even that old
Shadowpaw was surprised to see her mother, Night told her she was mauled by foxes. At first, Breezewhisper didn't know what Shadowpaw was talking about when she claimed she was her daughter. Then, she finally remembered her daughter that she abandoned for the sake of her clan.
She didn't remember her... even though she has two living kits from the same litter... Memory loss??
Breezewhisper was glad to see her daughter alive and healthy, yet frightened. StarClan was still sending her blood-filled visions about how dangerous Shadowpaw was.
She didn't remember the cat that haunts her nightmares? 😂
Of course, that meant Breezewhisper had to tell the clan that Shadowpaw was her daughter, and that she was alive.
Uh, did she though? Was there ever really a reason for it to be announced she was the evil Kit that needed to be abandoned or whatever?
PineClan was shocked to hear the news, especially Shadowpaw's littermates, Owlscreech and Blackpelt, who were already strong warriors.
Wait, had she not been talking to them? I thought she would've mentioned it to them or tried to make contact by now

Um, there's no other backstory so-

Review: That was Shadowstar. Uh, well I'm gonna start with the beginning. So, first thing you could consider changing is her name. "Shadowstar" is just a very basic name that kind of makes your character automatically sound cliche. Her name could be changed to something else resembling Shadow, but she also needs to be given a Warrior name. Everything past that was mostly okay, but she needs to be aged up to be leader, and her personality could be expanded on. Then, we get to the backstory, which is... less than explanatory. It starts off okay I guess, although I would've liked the weird prophecy thing to be explained (I also don't really like that plot point in general though because it's pretty common from what I've seen) but overall it's fine. Before getting to the major issues, I would like to point out it's odd Whitestar was in complete denial about her being the lost kit... for some reason. And to me it doesn't make sense that her mother doesn't remember her even though she's fairly young. Anyways! The problem comes in when the backstory just... stops. So, what happens? It ends on her mother announcing her as the kit she abandoned but nothing after that. It was never explained how she became leader, met her mate, or how she interacts with her littermates. There's nothing there so I can't really judge this Oc properly.... So, uh. Finish the backstory!!