It's a relatively obscure PS1 game in the horror genre.
The main thing that made it relatively unique among the horror game of the PS1 era is its lack of action mechanics. It’s essentially a horror dungeon crawler without action mechanics. You can run away or sneak past the invincible enemy, or if you gain a companion, the enemy kills your companion allowing you to run away when caught.
Game mechanics and ending spoilers
Essentially, your companion is your extra life. Different companions also have different abilities. Different endings result from who your companion is, or not having any at the game ending.
Its mechanics is more akin to Clock Tower—first person point and click at certain rooms, while being a first-person dungeon crawler in most other areas.
From the wiki article:
The game uses a first-person perspective, very similar to first-person shooter games, only without any means of combat. Throughout the game, players must travel through different areas of The Mesh and beyond, and must solve puzzles to progress to the surface. The player encounters only one type of enemy, and that is the mutating monster called The Hybrid. The only possible way for players to survive is to run away. Any close contact with the enemy will result in the deaths of their companion (and finally themselves). Once the companions are attacked, they are killed off permanently.
It is pretty different from the other horror games from the PS1 era, which made it relatively disappointing for those expecting it to be similar to the likes of Resident Evil or even Silent Hill.