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 knifing in 2203 and gun shot 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 suffers a heart attack and dies instantly, so it would appear that steroids and amphetamine derivatives are particularly harmful for him.
This heart is also possessed 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, so 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 ancient and partially damaged Auto-Doc services at 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 the Chosen One because he is "sick of this place." If the player is a slaver, Childkiller, has less than 4 Intelligence or has Karma of -101 or below, Cassidy will refuse to join them. He won't leave if the player gains these reputations or lowers their Karma after he has joined, however.
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 the Chosen One treats him with other drugs and consumables (stimpaks, alcohol, Nuka-Cola etc.) but they are perfectly safe.
Heart pills can be found in the 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), and is a pretty good shot with pistols, too. He can also use brass knuckles and spears. He claims that he used to hunt with a spear, but is a little ashamed that it makes him seem like a tribal. At this point, the Chosen One can remind him that s/he is a tribal, after which Cassidy will sincerely apologize and claim that spears are fine weapons used by many of his tribal friends.
He's an excellent shot and the best marksman the player can have as a companion. He's extremely effective with the Pancor Jackhammer and the M72 Gauss rifle later on. He can also be equipped with the Solar Scorcher, which he can use with about the same accuracy as small guns. It must be noted that he lacks the animations for large energy weapons, preventing him from using the plasma rifle, laser rifle or pulse rifle.
He despises ghouls (like Lenny) or chem inventors (like Myron).
Cassidy was supposed to have a talking head in the game. Black Isle Studios cut it from the definitive version, but it can be seen if the Restoration Project patch is installed. Notably, it showed neither the missing eye nor the metal plate.