Sergeant Johnson was the pal of Johnny and Arby, and he formed the third piece of their trio in their efforts to stop the Prophet of Truth and later the Gravemind. Before this, however, he was just another UNSC soldier, but he did take part in the death of Tartarus. He also has a habit of saying some funny lines, especially when it comes to speeches.
He started out as a Sergeant on Installation 04 and aided Johnny where he could. However, he died during the attack on Captain Jacob Keyes and his squad. Except... he didn't die, as such, something... happened.
Johnson was infected by a Flood infection form but because Johnson is actually a direct descendant of Mr. T the infection form withdrew from him and died, but not before injecting him with some elements of Flood DNA, which fused with his own to give him amazing regenerative capabilities and superhuman strength.
He later realised that he could not die, and when Installation 04 was destroyed he was battling a Zealot over an Assault Rifle, possibly Johnny's Magical Assault Rifle. When Halo blew up he jumped onto the back of the Elite and rode it all the way out into space until he eventually happened upon Cairo Station. With the nature of his escape from Halo remaining classified by the UNSC, Johnson was awarded every single medal Lord Hood could pull out of his ass, even the Purple Heart, which normally only Elites get because... well... their hearts are literally purple.
Johnson helped during the defence of Earth but, despite his invincibility, he developed a nasty habit of suddenly deciding he wanted to be where Johnny wasn't, and then running away on a Pelican. He escorted Miranda Keyes to Installation 05 and aided Johnny there too, and apparently single-handedly fought his way through a Flood-infested Library with Miranda in tow, a feat only Gods have achieved.
He was captured by Tartarus and taken to High Charity, then to Halo's control center. Bad move. Johnson escaped and somehow managed to aquire a Scarab. Bruties on the ring at the time report seeing a Scarab being spawned from the ground by a Shaman, this is actually Johnson. He formed an alliance with the Arbiter and blasted his way into Halo's control center. Arbiter started explaining to Tartarus why the Halos were deadly, but Johnson couldn't resist the urge to blast Tartarus with his beam rifle, setting the monkey's fur on fire. Arbiter then grabbed the Fist O' Rukt and hammered Tartarus's head in, as Miranda deactivated the Halo.
After this battle Arbiter scolded Johnson for attacking Tartarus prematurely. He then preceeded to drain Johnson's soul with his Uber Arby Powers but was stopped by Miranda, who offered a truce instead. Because of the interupption of the drain, Arby gained some of Johnson's invincibility and Johnson lost some, meaning that both of them could be killed but would simply enter a coma-state, spontaneously jumping back up again after a while.
He is also a crack shot with the Spartan Laser and helped during the battle with Guilty Guilty. He is mortally wounded during the battle, but since he is invincible he did not die, he just went into a coma. He apparently escaped the destruction of the Ark by riding the shattered remains of Guilty Guilty. The two apparently settled their differences and now travel the Galaxy together searching for Johnny and Cortana.