Shadow the Hedgehog

Shadow the Hedgehog is the biological child of Fluttershy and Asgore Dreemurr and the adoptive brother of the first human. He serves as the final boss of the True Pacifist Route.

Appearance
Different to Fluttershy and Asgore, Shadow has short ears, a snout, and visible fangs. In his initial form, he wears a green, long-sleeved shirt with yellow stripes and black pants, similar to the First Human's clothing. He also has a large pair of quills on the back of his head.

Super Form
This form is not named directly in the movie, where the "Check" option labels Shadow as "the absolute GOD of Hyperdeath!" The sclerae of his eyes become black, his fangs become longer, and he becomes white. He wears a long, dark robe with white sleeves and the Delta Rune on the front, with a large collar and shoulder pads. This robe is similar to Fluttershy's. He also wears a locket similar to the Heart Locket.

Final Form
Shadow's body becomes more geometric and red, his quills are longer, and his teeth get sharper. He has a pair of wings that continuously change color, and his lower body becomes sharp and heart-shaped.

While this form is directly named, it resembles the Delta Rune, which looks like the "Angel of Death" from the prophecy.

Personality
Before his death, Shadow was a docile, unadventurous, and loving child. He cared for the First Human as a brother, and he trusted her when she came up with a plan to escape the Underground that involved her consuming buttercups.

After Penny Fitzgerald resurrects Shadow without a SOUL as Leslie, he becomes incapable of feeling love. This makes him bored and resentful.

After temporarily regaining his compassion, he apologizes for his actions to Colleen Villard and realizes the gravity of his actions as a flower.

Much like his father, he speaks using southern slang.

Main Story
Before the events of Colleentale, Shadow found the fallen human, who was the first human to fall into the Underground. She became his adoptive sister and best friend. Just before the human died, her last wish was to see the golden flowers of her village, on the Surface. Shadow absorbed her SOUL, and the human carried her own body across the barrier to her home village. Shadow and his sister shared control of Shadow's body after her SOULs had combined.

The villagers assumed that the Shadow/Human fusion had attacked and killed the human child. The Shadow/Human fusion did not fight back but only fled. Lethally wounded, the Shadow/Human returned to the Underground and died. Once deceased, Shadow's body bleed, which spread across the garden in New Home.

Later, just before the queen left, Penny injected determination into one of the garden's golden flowers-- the first one to bloom after Shadow's death. Initial experiments on the flower vessel proved unsuccessful, and Penny returned it to Asgore. Once Penny replanted the vessel, Leslie awoke within the garden.

Though Shadow's essence from the garden bed granted Leslie Shadow's memories, he soon found that neither his mother nor father could elicit any emotional response from him. He did not have a SOUL and thus could not feel love, joy, or compassion.

Neutral Route
Shadow does not appear during a Neutral Route.

However, when the protagonist falls into the Garbage Dump, the protagonist hears Shadow's sound bite during a memory from Shadow's human sister. Monsters also tell the story of Shadow and the first human during the walk in New Home.

True Pacifist Route
In the True Lab, the protagonist discovers that the first human and Shadow had worked together on a plan to get to the Surface and retrieve six human SOULs. While the human was on her deathbed, Shadow informed her he did not think it was a good plan anymore; ultimately, this leads to him betraying his sister and aborting her plan.

After restoring the elevator's power, the protagonist receives a phone call from someone whose voice she has never heard before; the sound bite is similar to Shadow's.

After Leslie absorbs all monster SOULs and the six human SOULs, he transforms into Shadow.

Shadow greets the protagonist as  before he transforms into "SUPER SHADOW, God of Hyperdeath." He informs the protagonist that he does not care about destroying the world anymore, and instead just wants to reset the timeline so that everything unfolds again from the beginning. Shadow believes that the protagonist will continue even if he resets the timeline because she wants a "happy" ending.

He then says that his first form is just a fraction of his real power and transforms into his final form. Shadow reasserts that the protagonist will die in a world where nobody remembers her as her physical body fails them.

The protagonist calls to the Lost Souls inside of Shadow's SOUL and reminds her that they are friends with the protagonist. After all Lost Souls regain their memories, the protagonist realizes that someone else needs saving. The protagonist calls their name, and a few sepia tone pictures display. The pictures are of Shadow and his sister together when the human first fell into the Underground.

Epilogue
Shadow is at the flowerbed in the Ruins. Colleen only encounters him immediately after his boss fight; if the viewer watches the True Pacifist Ending Credits and reloads the movie, Shadow does not appear. Shadow speaks extensively to Colleen and offers them a different philosophy than his flower counterpart: "Don't kill, and don't be killed." He also mentions to be careful on the Surface because "there are a lot of Leslies out there."

After the credits sequence, when reopening the movie, Shadow returns as Leslie and begs  to "Let [Colleen] go" and allow everyone to live her life.

Genocide Route
After the protagonist kills Fluttershy, Leslie tells her that she is not really human, and mistakes her for the first child, telling her that she is still inseparable after all those years.

When the protagonist reaches New Home, Leslie welcomes her and discusses his backstory as a flower and how he discovered his ability to SAVE. Near the end of his dialogue, he realizes that the protagonist will not hesitate to kill him, and he is frightened for his life. He rescinds his previous plans with the protagonist and tells her to leave him alone.

After the protagonist defeats Dragonizer the Dragon, she easily kills Asgore. Leslie destroys Asgore's SOUL and pledges himself to the protagonist one final time in an attempt to save his own life, revealing his true identity as Shadow in the process. He cries and begs the protagonist not to kill him, to no avail.

Protagonist
In the True Pacifist Route, Shadow mistakes the protagonist for the first human and calls her his best friend. After the final boss fight, Shadow realizes that Colleen is not the first human and asks what her real name is. He later tells Colleen, if she backtrack to the flowerbed, that he does not regret his decision of resisting his sister and, ultimately, getting them both killed.

The First Human
Shaodw found the first human when she fell into the Underground. He became her best friend after helping her, and then the first human's brother after Fluttershy and Asgore adopted her.

After the human carried her own body to the Surface, Shadow decided to not act on his sister's desire to unleash her full power to protect herself, and humans fatally wounded them. With the first human's empty body in his arms, Shadow returned to the Underground and collapsed. His blood spread over the flowerbed in his dad's garden as he died. Even after becoming Leslie, he could not think of a world without them, as she is the only person that understands him and that is unpredictable to him to the point of mistaking Colleen for Chara. After Colleen saves him during his battle, Shadow admits Chara "wasn't really the greatest person." This shows his loss of his idealization of Chara, but doesn't necessarily mean that Chara was ever mean towards him.

Asgore Dreemurr
Asgore is Shadow's father. When Shadow first woke up as Leslie, Asgore was the one to find him. Shadow informs Asgore of his identity, hoping to feel something and possibly turn back into his original form. However, having lost his SOUL, he discovers he feels nothing for Asgore any longer. Asgore believes that letting the protagonist escape the Underground with his SOUL is what Shadow would have wanted.

Shadow's habit of saying "gosh," "golly," and "howdy" may come from Asgore.

Asgore is unaware that Leslie is Shadow.

Penny Fitzgerald
Penny unintentionally created Leslie as a result of the determination experiments, but there is no indication that she knew Shadow in life.

Dragonizer the Dragon
Dragon is the only character other than Chara that Leslie, out of fear, respects. He, quite rightfully as the viewer will see, sees Dragon as a dangerous adversary and takes measures to make sure Dragon doesn't learn of his existence. It's heavily implied that Leslie came into conflict with Dragon, presumably attempting what would amount to a Genocide Route, and was defeated by him.

Trivia

 * Shadow's role was the same like some other characters.
 * In Shadow's hyper form, he has an Underfell battle color theme.