Information from Pressure Wiki.

Sebastian Solace, also known as Z-13, "Handy Man", "The Saboteur", or simply "Sebastian" is a major character in Pressure, acting as an ally to the player as they venture through the Hadal Blacksite.

Sebastian runs a makeshift shop in which he sells items and supplies to prisoners in exchange for any research that they are carrying. He also supplies items to the player through Dead Drops, requesting kroner in exchange for items to be left at the Submarine Dock. Sebastian does not see convicts sent by Urbanshade as his enemies and is willing to help them in exchange for their help. The more times you exit and enter his shop, the more annoyed he gets, eventually kicking you out.

Sebastian can also be seen while the player is viewing Documents, being the one who shows the documents to the player. He will often reply with snarky or comedic comments about each monster. Interestingly, Sebastian has been forbidden from showing Mr. Lopee's document.

As of the Friendly Fire update, Sebastian is capable of harming/killing the player if one flashes him with a Flash Beacon. Upon using a Flash Beacon on Sebastian for the first time, he will grunt, and pickup said player before warning them "Don't do that again". Sebastian will then break the player's Flash Beacon before throwing them to the ground, dealing 50 DMG. if any player decides to flash him again, Sebastian will instead pull out a gun and shoot said player, instantly killing them. He will also tell said killed player that he warned them and does not show them the document on him.

According to his document, his first appearance in the pressure timeline is in 2013, when he was charged with murdering 9 people, and sent to execution by electric chair. Before that happened, Urbanshade ‘swooped’ him up, sending out a fake execution report, which ended up in Sebastian being declared dead.

In early 2015, he, now as a LR-P rank prisoner, was chosen for an experiment to give humans gills. He was tested on with DNA of a mutated angler fish, ███████, a blue whale, a great white shark, a silver spinyfin, a mantis shrimp and a sea snake. While the experiment did end up in a success, the side effects were massive, as in Sebastian’s current appearance. There was a chance at reversing the physical damage done, but it wasn’t necessary due to his low rank. Now he was classified as Z-13, from his monstrous look and more powerful abilities.

Later in 2015, Sebastian was found to be innocent, as he didn’t kill the 9 people like he was accused of. However, Sebastian wasn't informed of this information, nor was he released to the public. Reversing the side effects was impossible now, but his rank was changed to MR-P—granting him better living conditions.