Supernatural is a TV show that started with a significant death, and since then, no one -- not even the series' main two stars -- is protected from dying. The first scene in the TV show involves the death of Dean and Sam Winchester's mom and the episode ended with the death of Sam's girlfriend.

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The series then had Dean and Sam both die as well, plus angels, demons, and human allies have all perished along the way. With one more season remaining before Supernatural finally ends, expect more deaths to break the hears of fans worldwide. With that one final season still to go, here is a look at the 10 most heartbreaking deaths on Supernatural.

10. MARY WINCHESTER (S1E1)

It was the death that started it all. While Sam didn't remember his mon since he was a baby, Dean was old enough to remember her, and the death of Mary Winchester pretty much destroyed the life of John Winchester and set him on his mission to raise his sons as hunters and avenge her passing.

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When Dean and Sam finally met their mother years later after her resurrection, they eventually bonded with her and started a family they never knew -- even though John was gone by this time. When she appeared to sacrifice herself again, the boys faced a harsher heartbreak as it seemed they were never destined to be happy.

9. JESSICA (S1E1)

Sam introduces Jess to Dean when he invades their dorm at Stanford in Supernatural

Go all the way back to the first episode of Supernatural when Dean came to Sam for help when their dad disappeared. By this time, Sam was out of the hunting business and enrolled in college, trying to live a normal and happy life. That life involved a girlfriend named Jessica.

However, as the show has displayed throughout its run, Sam and Dean do not get a normal life, and neither Winchester boy gets a happy one either, with tragedy lurking behind every corner. When Jessica was plastered on the ceiling and burned alive, just as Mary was years before, Sam lost his only chance at happiness and ended up back in the hunter's life with Dean.

8. CASTIEL (S7E2)

Castiel looks down in Supernatural

Outside of San and Dean Winchester, Castiel is the third in billing when it comes to Supernatural's key protagonists. Castiel is an angel and a hunter, fighting alongside the Winchester boys against all forces of evil. However, Castiel has perished before -- as even an angel can die.

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This heartbreaking death came in the seventh season episode "Hello, Cruel World" and the episode began with a Leviathan possessing Castiel. However, since Castiel's vessel can't handle the Leviathan, it walks with Castiel into the water and then it explodes. Castiel's trenchcoat washes ashore, and Dean looks at it, realizing his friend is dead.

7. DEAN WINCHESTER (S3E16)

Jensen Ackles as Dean Winchester in Supernatural

Dean Winchester has died more than once. As a matter of fact, Dean has died 117 times on the show, although humorously, he died around 100 times in one episode thanks to the Trickster. So, out of the remaining real deaths, which one was most heartbreaking?

That would be in the season 3 finale, "No Rest for the Wicked." Having Dean die in the final episode of the season seems cruel, but Dean was dying through the season, and his deal with the Crossroads Demon arrived. When the episode ends, Sam is holding Dean, cradling his head in his arms, and crying -- a very heartbreaking moment. The fact that the last scene showed Dean ended up in Hell was even more startling.

6. KEVIN TRAN (S9E9)

Kevin Tran lays dead in Supernatural

Some deaths don't seem fair, and when they happen to a decent person, it makes it even more heartbreaking. Kevin Tran was just a regular kid in high school studying in the season 7 episode "Reading is Fundamental" when lightning struck him, and he developed the ability to read the Word.

Sadly, this makes Kevin a valuable asset for Dean and Sam and an enemy for beings who want to rule the world. By the season 9 episode "Holy Terror," Metatron sends Gadreel to kill Kevin, and Dean is left crying knowing that one of his closest friends died trying to help them.

5. JOHN WINCHESTER (S2E1)

Jeffrey Dean Morgan as John Winchester in Supernatural

Throughout the entire first season of Supernatural, all Dean Winchester wanted to do was find his dad, keep his brother close, and hold onto the only family he knew. While Sam and Dean struggled through the season, they grew closer than they ever did before. However, every time they got close to John Winchester, something happened to pull them farther apart.

The first season ended with all three Winchester's finally together when a truck driven by a demon smashed into the Impala. The first episode of season 2 sees Dean dying and both John and Sam fighting. By the end, John gives his own life in exchange for the demon bringing Dean back, saving his son with the ultimate sacrifice.

4. SAM WINCHESTER (S2E21)

A fatigued Sam Winchester in Supernatural

Much like Dean Winchester, Sam Winchester has also died multiple times on Supernatural. Out of the six times that Sam died on the show, the most heartbreaking one was the first one. This happened on the season 2 episode "All hell Breaks Loose - Part 1."

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This was the season where Supernatural fans learned about Sam being one of the Special Children, and he ended up battling the others to see who survived to serve Azazel, the Prince of Hell. The death was brutal, and unexpected mas Jake drove a knife into Sam's spine and twisted it. This death was terribly heartbreaking, as Dean held his brother's lifeless body and cried.

3. ELLEN AND JO (S5E10)

Ellen and Jo on Supernatural

Sacrifice is vital in Supernatural, and there isn't a stronger moment than when hunters sacrifice themselves to save not only the world but their fellow hunters and their loved ones. By season 5, Ellen and Jo had become valued allies, and Dean had developed strong, yet unrequited feelings toward Jo.

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The sacrifice came when hellhounds closed in on Sam, Dean, Bobby, Ellen, and Jo. The end starts when a hellhound shreds the left side of Jo, and everyone goes into hiding. Jo knows she is dying, and Ellen makes the call and decides to stay with her daughter, taking her own life in a massive explosion to kill the hellhounds to save her friends.

2. CHARLIE (S10E21)

Charlie Bradbury on Supernatural

Charlie Bradbury was a fan favorite, partially because Felicia Day portrayed her and partly because she was just a great character. She arrived in season 7 as a former employee of Richard Roman Enterprises who turned on them when she realized the Leviathons were taking over.

She became a strong ally to the Winchester boys and a friend who was there to help them any chance she got. Her final act was to help them came when she attempted to decipher the Book of the Damned in season 10 and ended up dead at the hands of Eldon Frankenstein but not before succeeding in her task.

1. BOBBY SINGER (S7E10)

Bobby Singer in Supernatural

Bobby Singer was John Winchester's closest friend and someone who took his job of watching over Dean and Sam Winchester very seriously. He was there as their closest ally in the second season and helped them become stronger hunters throughout the rest of his life.

Heartbreakingly, his life came to an end in the seventh season of Supernatural, "Death's Door." Bobby had gotten shot in the head by Dick Roman. He ends up slipping into a coma, and the episode plays out in his mind before he finally wakes up and calls the boys "idjits" before passing on in a moment that destroys the Winchesters.

NEXT: Supernatural: The Top 10 Heroes That Need To Be Brought Back For The Final Season