Guardian One is a Guardian-class submersible, built by Nankatsu Industries and used by Macallan, Anton, Tulley, and Harlequinn to escape the Nankatsu Submersible Proving Grounds. It is a deep-sea research vessel capable of cruising at extreme depths.
The hull of Guardian One is made up of an organic substance made by Nankatsu that gets stronger as more pressure is applied until, that is, it reaches its breaking point. There was no reference to a maximum operating depth on board, which means that Nankatsu has either not found it or it is irrelevant (meaning it could go as deep as the oceans allowed it to).
Guardian One is unarmed but is packed with stealth and defensive capabilities, one of which is a massive "sonic ink" burst that is meant to destroy the tracking computers on incoming torpedoes and another one of which is a massive water ejection system capable of sending the submersible a half mile in under ten seconds, inspired by jellyfish. Its lighting system is primarily bio-luminescent bacteria, which glows when given a mild shock and so is unhindered by the crushing depths of operation that the submersible faces.
Guardian One also has Mechs, or human-shaped robotic exoskeletons that allow the operator to conduct research or sampling at extreme depths by morphing the pilot's every move identically.