Humans

Will

The main character of the series. He started the webseries in order to play the game his sister left him in her dying months.

Childish Older Sibling: Implied to be this (or at least be seen as this), despite he and Teddy being twins.
Determinator: Despite his countless failures in the game, he never quits playing.
Mirthless Laughter: Understandable, given how insane the game can get.
Successful Sibling Syndrome: Averted. In the game, Teddy said that she was worried that her various successes would cause Will to feel this way, but he immediately denies this when it comes up.
The Unfavorite: Shows shades of this, especially in Act 1, Episode 16. Will and Teddy's parents see all of his hobbies as lesser when compared to Teddy's achievements, even though he's a straight A student.

Will's Sister

Will's deceased twin sister. She's the creator of Diminish, a game tailor-made for her brother. Her name is Teddy/Theodora.

Academic Athlete: On top of making a functional, detailed game in the span of 4 months, she was training to be an Olympic swimmer prior to her death.
Broken Bird: As the game was created prior to her death, it's almost a given that most of her thoughts are on the negative side.
Cool Big Sis: While Teddy and Will are twins (with Will being born first), Will still sees her as this.
Extreme Doormat: Downplayed. Teddy sees herself as such, and worries that this will have major consequences after her death. It's unclear if she was a doormat, or this view was a product of her impending death.
The Gift: Insanely talented. High academic scores, high athletic scores (given she was accepted into the Olympic swim team), and a talented artist and game designer to boot.
Parental Favoritism: On the receiving end of this. An in-game text conversation in Act 1, Episode 16 shows a conversation between Teddy and one of their parents about Will. The parent praises Teddy for all of her accomplishments while dismissing all of Will's hobbies as lesser. Teddy is clearly uncomfortable with this treatment.
Posthumous Character: Dead at the start of the series. All we know about her comes from the game's text and Will's ramblings about her.
Secret-Keeper: Has a horrible secret that she has been keeping from Will. What it is is implied to be rather gruesome, given the horrific imagery that usually accompanies any mention of it.
Tomboy: Hinted at by Will.

In-Game Characters

Apollo

The main character of the game. They are the immortal Olympian themself.

Conditioned to Accept Horror: On the first playthrough, they constantly zigzag between this and despair. Played straight on the second playthrough, where their "death" counter turns into a "tries" counter.
Deadpan Snarker: After a while of dying, Apollo starts to snark on Will's ability to play the game.
Determinator: Despite all the pain Apollo goes through, they never give up their mission to help other people. Exemplified in Act 1, Episode 9 where, after being stuck in a constant loop of dying due to never saving at a checkpoint, they force their way out of the loop by using pain as a source of power. In the process, this changes the "death" counter into a "tries" counter, showing their new-found strength.
Fights Like a Normal: Attacks by punching. Odd, given that the Greek god is associated with an enhanced bow and arrow.

Justin

Let's shoot for one hour, yeah?

A Skull that chases down Apollo in the later parts of the level

Dem Bones: He's a talking skull.
Friendly Enemy: After a couple of deaths, he begins to form a friendship with Apollo. Even Apollo cheers him on to do his best.

Unknown Voice

A voice that follows Apollo around. Little is known about them.