Junk is the moniker of homemade computer made of various computer parts that acts as medium for Hyper Agents to communicate with his chosen hosts as well as means for his allies to aid him in battle.
In Denkou Choujin Gridman, Junk is a homemade computer created by the three friends Yuka, Naoto, and Ippei. Following its completion, Ippei used it to design an ideal superhero which he named Superhuman Lightning Gridman for fun. Little did he knows that an interdimensional being amd Hyper Agent whom coincidentally shared his name with that of his superhero design, would later use it to assume a new default form before recruited Naoto as his partner and host to stop Khan and Takeshi's digital kaijus from hacking electronics through Computer World. With Khan's threat increases, Ippei would upgrade the Junk and even design Assist Weapons with it to combat the digital threat.
In Superhuman Samurai Syber-Squad, instead of Junk, the computer that acts as medium for the Hyper Agent and his chosen hosts and human allies is an ordinary computer that belonged to Sam Collins. Not long after chosing Sam as his host, Servo (Gridman's counterpart in the American series), left some programs behind for Sam's friends to access Assist Weapon for them to assist him and his host combating Megavirus Monsters.
In SSSS.Gridman, Junk is once again, old computer used by Gridman as medium to speak to his chosen host Yuta Hibiki and later, his fellow Gridman Alliance members Sho and Rikka. Here, it is a homemade computer which is among items sold by Rikka's Mother that Gridman used to call out Yuta for him. Initially, only Yuta who can communicate with Gridman as others only see inactive screen, but after his first transformation and battle, Sho and Rikka can perceive Gridman too, though only able to properly communicate with the Hyper Agent once Samurai Calibur comes along and optimized the Junk.