Jericho was born in 2212. He was once a dangerous raider out in the Capital Wasteland, who killed and stole for measly amounts of caps and loot. After fighting with the raiders for many years, and becoming a crack shot with a rifle, Jericho settled down in Megaton to enjoy his twilight years. But he didn't lose his raider ways. The smarmy, rude Jericho helps protect the town, but still gets drunk and curses at passersby. He's thought about getting back out into the wastes to fight again, but not without some caps for "equipment." Jericho occasionally does some odd jobs for Colin Moriarty.
Reading Moriarty's terminal suggests that Jericho may have once attempted to rape Jenny Stahl. Although this cannot be brought up in conversation, Jericho and Jenny may exchange dialogue that can relate back to this.
If you kill all of Megaton with him as your follower he will not become hostile towards you. However, if you kill the brahmin outside the clinic, or steal from his house, and then fire him anytime after that, he will become hostile.
If you decide to blow up Megaton during the quest The Power of the Atom while he is your companion (and will therefore not die in the explosion), and fire him afterwards, he will walk to the Megaton ruins and stay in front of the crater. He will mention the destruction of Megaton when you talk to him, but won't feel sorry for its residents. You can still hire him as your companion.
Jericho can be recruited as a follower, but you must have evil Karma for him to join your party. When asked to join the player's party, he requests 1000 caps for "supplies" before he will join. He will not quit if your Karma improves and can be rehired free of charge.
You can persuade Jericho to tell you a secret for 20 bottle caps. He tells you that Colin Moriarty keeps a key around his neck, which unlocks the upstairs locker in Moriarty's Saloon. Inside are all the caps Gob and Nova make, as well as a combat shotgun. However, the key was cut, and the locker can only be lockpicked. It has a Hard level lock.
Jericho can be saved from the destruction of Megaton by being hired beforehand and taking him to a safe zone outside of Megaton.
If you first speak to him while wearing the Vault 101 jumpsuit, he will call you a "pampered vault asshole" instead of a "wasteland asshole."
He will sometimes comment on how the collar of his armor is too tight.
If you recruit him again after Megaton's destruction, he will tell you he has begun to grow "a fourth arm" standing next to all the radiation.
He mentions that he is the best shot in town with a rifle and therefore the main defender against raiders along with Lucas Simms, Deputy Weld and Stockholm.
He can also be heard muttering that he's stepped in something and worrying about the stability of the floor you're both standing on.
For some reason, Jericho prefers the pre-War bonnet to virtually all other non-combat headgear.
Sometimes he will say "I've been here before," however this is just another line of his frequent banter and will say it regardless of where he is.
If Jericho survives the destruction of Megaton and witnesses the player kill Moira Brown at the Megaton ruins, he will turn hostile. If he is currently employed when Moira is killed, as soon as the player dismisses Jericho, he turns hostile.
He'll say some different quotes if you have him as a companion while your Karma isn't evil. One example is along the lines of "What's going on kid? I thought you were supposed to be some kind of bad ass."