Madeline is a young red-haired Canadian woman.
She is determined to climb Mount Celeste, despite opposition from others including herself.
She is the only playable character in Celeste, apart from being the main character.

Theo is an American man of Brazilian descent.
He is also climbing Mount Celeste, though unlike Madeline he isn't aiming for the summit. Compared to her, he is more laid-back and doesn't have the same internal struggles--however, he does have a reason for leaving his job to climb the mountain.
Theo appears time and time again throughout the game as a supporting character.
The player is able to talk to him each time and watch Madeline gain a friendship with him.

Badeline, referred to in-game as a Part of Madeline (Part of Me/Part of Her), is an aspect of Madeline's mind that gained her own body in Chapter 2.
She plays an antagonistic role throughout much of the story, chasing Madeline and breaking gondolas in hopes of stopping her from climbing the mountain.
Se puede jugar con ella, pero solo a través de la opción Otro Yo en el Modo Variante.

Granny is one of the few people to live on Mount Celeste.
She has a cabin at the very base of the mountain and appears occasionally to guide Madeline with her knowledge of the mountain, but how she manages to appear everywhere is unknown.
Madeline does not initially enjoy how she talks to her.
In Farewell, if you look closely at the picture Theo sends Madeline, you could read "Celia & Carlos" written in cursive, stating that her real name is Celia.

Mr. Oshiro is the deceased owner of the - now defunct - Celestial Resort and mostly appears in Chapter 3.
When Madeline comes to the resort, he is cordial towards her in hopes of getting her to stay there.
Granny describes him as a "lost soul".