Josh represents the heart. Josh represents the will to live.
Hear me out -
We've now had a car swerving to miss Josh in the road in two different videos. Once in Next Semester. Three times in HDS. HDS has Tyler alternating between being awake and asleep, as passenger and driver.
Car catches fire when it passes Josh and his drums, which are also on fire. Three times. Then explodes and Tyler escapes 'falling asleep at the wheel,' hops out of car to stand with Josh. Josh is the heart. The heart that makes the beat.
What does the heart do even when the mind is asleep? It keeps going. Keeping the beat. Josh represents finding the will to live. Josh is the flame found through the heart. Fire. Torchbearer. Life. Staying alive.
That's why Josh can be in two places at once, and Tyler has to seize to do the same. Tyler is the mind finding ways for the fire, the torch, the will to live, to break in and burn.
Overarching Story of Dema, Trench, and Clancy - Theory / Insight
I haven't listened to Twenty One Pilots this deeply since middle school. With the new album coming out, I decided to look more into the lore of Tyler's universe, and I have a theory on the overarching story; what the story could representing.
TL/DR at the bottom
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I view all of this as representation of feeling trapped by mental illness. When you run, it pulls you right back. The Bishops represent the struggles that come with mental illness, or those habits that bring you back. In the Jumpsuit music video when Clancy is caught, Nico marks him with black paint before trying to take him back to Dema. The black-out paint on Tyler/Clancy's neck and hands from bits of Trench and all of Blurryface may represent that weight of mental illness, along with the labels and marks coming with it.
Clancy tries to run, but he can't escape. Nico will always find him. Scaled and Icy may represent the mask put on to hide everything going on in his mind.
Now, in the new album and next story arc, Clancy is returning to Dema and therefore, facing the Bishops. He is facing his demons. He is facing his mental illness. He's learned he can't run, but he can fight back.
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TL/DR: The Trench and Clancy story may be a commentary on mental illness and the inability to run away from it, but the ability to fight back and conquer it. OR I could be projecting.