Nakagaki is a hiker and minor character in The Enigma of Amigara Fault.
After making his way to Amigara Mountain, Nakagaki announces that he had found a hole that matched his shape, and without any warning, removed his clothing and stepped inside. Despite a team of scientists attempting to enter to rescue him, Nakagaki was well and truly lost.
That night, Owaki has a nightmare in which he sees that Nakagaki can't continue any further into the hole, as the earthquake which revealed the fault in the first place had deformed his tunnel.
Nakagaki presumably died of dehydration as a result of this.