Observer

Observer is a TESTBED tank used for spectating the game.

Design
Observer appears as a almost transparent tank with no weapons.

Technical
The observer moves extremely fast and has a large field of vision. The tank does no damage and passes through everything. It cannot die unless hit by a sandman bullet or killed using commands.

Upgrades
Observer upgrades from TESTBED

and can upgrade to:

Boss Observer

Auto Pepsi

Trivia

 * It is possible to get assist kills by ramming into a boss or tank as it dies. Even though observer does no damage, the fact that it is technically attacking through ram gives it that kill. This could be its only way to get score.
 * As of the PATCH 10/23/2020 (2), Observers are now unable to push stuff and aren't affected by tank abilities.