"I wish I could play outside like the other children..."

Alessa Gillespie is a main character in the Silent Hill series. She appears in the following games from the series:

Silent Hill
Silent Hill: Play Novel
Silent Hill: Moblie
Cage of Cradle (cameo)
Silent Hill: Origins
Silent Hill 3 (mentioned)
Silent Hill 4: The Room (mentioned)

Alessa is the first character to ever appear in a Silent Hill game. She is the centre of the main story, although she doesn't appear in some of the non cannon games (like Silent Hill 2, 5, 6...). But we will also discuss what is her influence in all of Silent Hill in the in depth section ;D

She was the daughter of the High priestess and leader of the Order, Dhalia Gillespie, who wanted Alessa to sacrifice herself for the Order. The Order is the religious cult of Silent Hill.
Dhalia needed Alessa to host God, for it to be reborn. But Alessa was clear to deny all her petitions. After Dhalia sets her on fire at their home, in order to do a ritual in which she would summon God in Alessa, she survives (thanks to being pregnant with God), but completely burnt. Dhalia keeps her alive in a basement waiting for the day they can revive their God and go to the paradise they believe in.

Truth be told, Alessa was the trigger to the hell later unleashed in Silent Hill. Alessa was born with supernatural mental powers (telekinesis, levitation, premonition and astral projection), which led to her being bullied, treated as a witch and rejected during her days at the Midwich Elementary School. The reason why Dhalia thought it would be a good idea to summon God into Alessa is the fact that she has amazing powers like no one else in town (although this ritual had been performed in other girls before, no one was as strong as Alessa).

After all of that, years went by and Alessa was no longer a teenager. Her wounds healed but deep and permanent scars and burns covered her body. That's where Silent Hill goes wrong... Alessa's manifestations of hate, hurt and darkness create the Otherworld and Fog Silent Hill.
This is a quite messy introduction to Alessa that I hope to clear up whenever I keep writing the in depth essays. Please stay tuned!