This Oc belongs to Bellaisthebestrus

Name: Quailheart
Sounds nice, and "Quail" isn't overused

Past names: Quailkit, Quailpaw
Makes sense

Name reason: Quails are known to be small and have a brown plumage (color of feathers) which Quailheart literally is small and brown.
A very literal name, but I like it
Quailheart means that he has the heart of a quail. Quails are usually heard but not seen and Quailheart is a voice of reason but doesn't like to take all the clout from the other clan members.
"All the clout" ??
So he's more of an introvert but loves to help his clan.
The reasoning behind "heart" is good

Aprearence: He is a brown glossy tabby. He's little and skinny. Has white fur around the muzzle and down the stomach. His tail has a white tip at the end and if you look closely he has darker brown streaks coming from his eye to the side of his face. (Quails usually have long stripes from there's eyes) He has a few scars on the back of his neck because in fights cats go for the back of his neck to bite. He has yellow eyes coming from his mother's side
Alright, pretty neat description (ew, who says that? 😂) but I'd consider shortening it with commas into a few sentences.

Gender: tom
Mmhmm

Age: 78 moons (6.5 years)
Oh, wow! He's older than a lot of Ocs

Personality: voice of reason, learns quickly, loyal, blunt, introverted, seems too serious, not the best at dealing with loss,
Okay, seems pretty good
and a lil submissive.
Uh, wow- Is it hot in here?
😅🥵😳

Sexuality: big gay
We love that 😌👌

Clan: WindClan
Makes sense why he's named "Quail"

Rank: Warrior
Figured

Mother: Mapleblossom, has silky and glossy dark brown fur. Her eyes are usually in slits that are as yellow as a sunflower.
Mapleblossom is officially the most beautiful name ever and I wish I could steal it（≧∇≦）

The description is nice, thought it could use something extra

Father: Stormfall, A raven black tom that has scars all around his body. His tail has the most scars almost making it look bent. He has cloudy blue eyes.
Does he have cataracts?
Though he pushed him out of his life when he agreed that his brother should be kicked out of the clan.
Okay, so not so nice

Brother: Coal (Coalpaw) a raven black tom just like his father. The name pretty much explains itself. Became a rouge after he was kicked out.
I was right! Though how is he named "Coal?" Unless he was rogue or something before.... becoming a rogue again??
Him and Quailheart used to have a close bond. Always doing clan activities together. He has pale blue eyes. Used to be an extrovert and had the determination to become leader. It explains more of his story in the backstory.
Kk, I'm waiting for it 👀

Crush: Hero
( ^ω^ )

Mate: Hero: a big and broad red/ginger-ish tom.
Uh, Ginger is a red color? Do you mean like a bright ginger, or a russet color?
He's a loner and they meet on Quailheart's adventure to find his brother. He has emerald bright eyes that Quailheart is drawn to.
I suggest re-writing that as "Bright emerald-green eyes" so its easier to read.
He is also drawn to Quail and just wants to help and protect him. Used to be in a home until he wasn't properly well fed and decided to live in the woods to hunt on his own. (Named him hero cuz he kinda died as a hero.)
So, what was he called before that? His name couldn't have been Hero for "dying as a hero" when he was alive. To make more sense, I'd just have his Twoleg given name be Hero

Kits: Nope
Okay

Mentor: Oaktail, a calico she cat with scars on her eyes. Her eyes are a dark ugly yellow and her tail is fully ginger. She taught him well though at times when she was in a bad mood she would be rough and angry.
Ooh, I love the name Oaktail. Her description is fine too.

Apprentices: Frostfur, A pure white fluffy she cat that has tufty.
"Tufty"? I assume you meant tufted ears
She trained well and is now deputy which is good for Quailheart.
Good for him, because now he has favor of a high-ranking cat?
Frostfur also has wide amber eyes. One of Quail's bestest friends.
Hmmm.. She has a canon name, so I'm not sure about that, but I do like her amber eyes
Berrypaw. A timid she-cat with matted yellow fur. She also has little ginger freckles around her eyes and has a white underbelly.
She has a very cute description! But what's her eye color?
She is very shy and lacked in her training. When Quail died she had to get another mentor named Thistleshine
Wait, he died? Why doesn't it say on the Age "78 moons at death" ?

Friends: Frostfur (description in apprentice section)
Yup

Flamenose. A ginger tom with a surprisingly bright orange muzzle and green eyes. He's out going and really wants Quail to appreciate himself more at one point he had just a little crush on Quail until he realized that Quail wasn't interested in him.
Ok, he's pretty cute. I like him 😌

Coalpaw (his brother/ description in kin section)
Right, but isn't his name just "Coal" now?

Backstory: Quailpaw always wanted to be just a normal warrior.
Straight up skipped the kithood XD
Though his brother had other plans. Coalpaw would always rant about being a riotous leader
He wanted to be a disordered leader?? That's just the definition I got from google
and would look up too all the successful warriors. Quailpaw would cheer him on every time Coalpaw would pretend to be leader.
Sounds like Nursey games to me
But he would always see the medicine cat, Roseleg glaring at Coalpaw.
I'm curious to know why...
He always blew it off until after training there was a clan meeting. The leader stated that in order for the clan to survive Coalpaw must be banned from WindClan because StarClan says he will bring danger to all clans.
Uh, so he's like evil I guess?
Quailpaw was caught off guard by this and looked around the clan. The clan members were also caught off guard but started to agree with the leader. Even his father yelled remarks about Coalpaw being a burden.
Damn, that's just cold af
Their mother just stayed quiet and looked at her paws. Coalpaw was petrified of all his clan members agreeing he should be kicked out. In terror he ran out of the clan while tears were pouring down his face.
Sad boi
Quailpaw couldn't stand it without his brother and ran away to find him. Day by day he ran across rivers and pounds,
So... he ran across rivers and... Ran into pounds? Am I hearing that right? 😂 Oh, maybe it was supposed to be "Ponds"
and searched in every nook and cranny. On his way he met a young cat named Hero.
Yay, love interest
Hero fascinated him with his stories and great techniques while hunting.
But... He's a loner. Shouldn't Quailpaw already know all this stuff?
They grew closer and closer on their journey. Finally they found his brother but he was already crushed to death by a tree branch.
JESUS WHAT A HORRIBLE WAY TO DIE
He was horrified by this and Hero comforted him as much as he could. Quailpaw still didn't know why StarClan would do this. Quailpaw sighed and decided he should go back to his clan. Hero insisted he would go with him. As they were almost near the clan and end of their journey they saw two kits backing away from a vicious dog. Quailpaw recognized both of them as Shrewkit and Tulipkit.
Random kits he has to save... nice
Hero and Quail both jumped at the dog. Before the dog died from blood loss from its wounds it bit into heros stomach instantly going through his flesh with its freakishly long teeth.
Uh... You're telling me that two cats could kill a whole dog by themselves, but weren't smart enough to like, move away??
Quailpaw could almost notice Hero giving a gentle smile and mouthing I love you to him.
I'm sad but they're also like... really young to be in love
Quailpaw shrieked as Hero went down with the dog and even the kits yelped. Quailpaw tried as much as he could to do anything to save him but there was no use. Quailpaw hesitantly picked up the kits by their scruffs and ran to the clan.
How was he trying to save him if he didn't immediately get the medicine cat? Seems a little odd...
Quailpaw trembled while he walked into the clan with the two kits. The clanmates were so grateful and pleased with him that the two kits were safe. The kits ran to the nursery den to their mother. Quailpaw was surrounded by the clanmates he knew and loved. He learned that the kits were out because they wanted to find him even though they were told not to. 
Aw, that's pretty cute
The leader gave him his warrior name because of his courage and bravery when he saved the kits.
Okay, but I thought it was for him being soft-spoken but also logical. His parents and clanmates cheered his new warrior name. Quailheart.
Good for him, but where's the rest of his backstory? We should see him until he dies...

Other: He died of kidney disease (pretty sure that's important, and also sorry if my grammar is bad)
Yes, thank you for including that XD And it's fine

Review: Alright, so that was Quailheart! I think overall he's an ok character, but something about him just seems.... boring XD Maybe I'm just biased because "Quail" is a very boring prefix to me personally, but I think maybe his friends and backstory could use a little work. Like, I think he should have more friends than his brother, apprentice, and one other cat (who we don't see in the backstory :/) because he was welcomed back as a hero (hahaha), so no one held bias on him anymore right? Besides that, on the backstory where is his kithood?? And why does the backstory cut off at him being welcomed home, but it says he dies at 78 Moons. If he's going to die the backstory should continue to that point, since we didn't see anyone listed as his friends except Coalpaw, who died. Just make the backstory longer I think, and maybe go into more detail about his relationship with Coalpaw or his other friends.