His long history of adventuring has left him in a less than perfect state, including a metal plate in his head, many old wounds, a missing eye, and memories of some particularly nasty injuries (including a kniving in 2203 and gunshot wounds from 2195, 2199, and 2201).
Despite his life of violence, he is a very kind and polite person, at least to people he respects. Anyone who gets on his bad side can expect to be treated no better than dirt, usually accompanied by a number of curses from Cassidy's sizable catalog of insults. Myron is the usual victim of Cassidy's picking, as the adolescent genius, apart from creating the pharmaceutical scourge of the wastes, is perceived by the old man as a piece of human trash.
With time, Cassidy began to suffer from rhythm and conduction disorder, some kind of block. He needs a cardio booster (cardiostimulator) shot to keep his heart beating. When treated with Psycho, Buffout or Jet, Cassidy dies from a heart attack instantly, so it would appear that steroids and amphetamine derivatives are particularly harmful for him.
This heart condition is also inherited by his daughter Rose of Sharon Cassidy in Fallout: New Vegas.
Cassidy needs real medical care, and Vault City is the only place to get it, some time before 2241, he came to the city. Unfortunately, as with any other "outworlder", he is not allowed to receive it. Like anyone else not allowed to enter in Vault City, he can pay for an ancient and partially damaged Auto-Doc services at the Vault City courtyard clinic, but basic treatment (healing wounds) is not enough, and more advanced procedures are prohibitively expensive. Also during this time, he acquired a bar named the Spitoon on the outskirts of the city.
When the Chosen One speaks with Cassidy, he can be recruited to accompany them because he is "sick of this place." He will, however, not join a slaver, childkiller, a low Intelligence character (IN < 4) or a Karma of below -100. He does not leave if the Chosen gains these reputations or lowers their Karma after he joins, though, until asked to leave and then come along again.
If the Chosen One can convince Cassidy to leave Vault City, then the only problem is Cassidy's drug intolerance. He expresses some doubts when treated with other drugs and consumables (stimpaks, alcohol, Nuka-Cola etc.) but they are perfectly safe.
Heart pills can be found on Westin's Ranch in NCR, but have no effect on Cassidy and are unrelated (though he claims that he 'feels' better immediately after taking one, he still regularly complains of problems).
Fallout: New Vegas confirms that he joined the Chosen One at some point on his travels, with his daughter Rose of Sharon Cassidy subsequently choosing to assist the Courier. Shortly after having his daughter he went East of California and vanished.
He declares that he is skilled with shotguns and rifles mostly (he begins equipped with a sawed-off shotgun), as well as pistols. He can also use brass knuckles and spears; he claims that he used to hunt with a spear but expresses shame about making him seem like a tribal. At this point, the Chosen One can state they are a tribal, after which Cassidy will sincerely apologize and claim that spears are fine weapons used by many of his tribal friends.
John Cassidy was originally named MacRae. This can be observed when selling him to Metzger and judging by his older VCMACRAE.MSG.
Cassidy claims his father named him after his favorite comic book character, which probably is Proinsias Cassidy, an Irish vampire from the comic book series "Preacher", created by Gath Enis and Steve Dillon. Cassidy uses the same swear words and sayings throughout the game as the comicbook character.
Rose of Sharon Cassidy sometimes says she could use a drink, just like John Cassidy.
The Fallout 2 Restoration Project adds a fan-made, improved talking head for Cassidy into the game. He is voiced by the Restoration Project's lead writer (who also voices him in the Mutants Rising mod).