The monster takes on a humanoid form. Interestingly enough, the monster's body did not enlarge or alter enough to shred its human clothes, which was a Hawaiian shirt, fisherman's hat, and red shorts. The monster also carried a bag of its former self's belongings.
At the onset of the Monsterfication Apocalypse, the Reach Monster, while human, and his son were attacked by an unknown monster (based on his journal, the monster could be the Speed Monster). The man's son was caught and killed by the monster. Since then, the man regretted not doing more to save his son. He had intended to pack his things and leave the apartment, as there was nothing left for him there, but his regret ate away at his will and he succumbed to monsterfication.
Wook Pyeon and Hyun Cha arrive on the 5th floor of the building to check for monsters. Hyun notices a hand clinging to a wall around the corner and realizes it must be a monster. He grows suspicious of their surroundings and is able to detect the monster reaching around from behind them with its other hand to abduct them. While it is able to grab Hyun, Hyun in turn stabs the monster's arms and electrocute it, though Hyun also gets shocked as well. This stuns the monster long enough for Wook to find the main body, rummage through its personal belongings, and incinerate it.
The Reach Monster is not a physically powerful monster. Rather, its unique ability, based on its human's regret over not reaching out to save his son, are extremely extendable arms. The arms are able to extend the length of an apartment hallway. It is unknown how the monster was able to detect Wook's and Hyun's exact location, as its main body's location made it impossible for it to see them visually.