It is regarded as a forbidden and sacred place and to enforce that no one reaches the peak let alone enter the mountain, the Serene Village adults usually have guard duty. Also, it holds a secret that villagers in question seem to be aware of, however they refuse to elaborate about it when asked because it is to keep the spring found at the mountain's peak a secret from evil. Even if one should get by the security to the mountain's peak, they would need to overcome a barrier to access the spring, which can only be broken by a Pokémon that was from the human world.
First Expedition
Nuzleaf gets the idea to visit the mountain as days go by and the idea of waiting for the player to remember something about their past life bore no fruit. Nuzleaf then proceeds to take the player and the partner to the side while elaborating of going to Revelation Mountain in secret, much to the shock of the partner. Despite this, Nuzleaf explains why he's going there: he believes that the mountain is somehow linked to the reason of the player being made from a human to a Pokémon, even if it is just a feeling. However, he does plan to retreat if his plan should start going south. Nuzleaf then invites the duo to tag along, to which they do so.
After completing preparations to go to the mountain, the trio then head off to the entrance. At the gate, Hippopotas is guarding the way. Nuzleaf tells the player and partner that the adults take turns guarding the mountain's entrance, with the player bringing up the question of what to do if that' is the case. Nuzleaf then proceeds to walk up to Hippopotas and tells him that there was a "mistake in guard duty", which Hippopotas buys and happily walks off, giving Nuzleaf and his posse access to the mountain.
Sometime later, after scaling half of mountain, the trio come across yet another gate, but with much fiercer-looking security than Hippopotas, this being a Poliwrath, officers Magnemite, Magneton, and Magnezone, and a Beedrill. The trio soon realize how pointless of a cause it would be to try to fight their way through them, and had no choice but to turn back, the expedition being deemed a failure as no answers were given to the player about their memory.
Back at the village, they express their disappointment and Nuzleaf then has to go tell Hippopotas that the mix-up was a mistake within itself.