Death's true form was heard in Abandon All Hope..., when he ascended from his cell, with tremendous wings convulsing. Now, whatever it looks like, only God, the other Horsemen, and the archangels have seen it.
Unlike his reapers, Death is dressed in a dark business suit, as opposed to mortician wear which the reapers (except Tessa and rogue reapers) are clothed in. He also carries a steel tipped cane and is indeed very thin like Bobby's wife described. He also appears to have overly large pupils that expand like the sockets of a skull depending upon his mood. His ring is pure silver and has a white stone set in it. Death also has a British accent.
Due to his advanced age, Death is completely detached from the rest of the universe. He even compares Dean to a bacterium. He cares little for major events of the world like the Apocalypse, and despite his coerced involvement by Lucifer, he only pays the events vague interest, resulting in his tendency to wander off from his assigned targets. Unlike the angels or Lucifer, Death considers the Winchesters and their involvement in the Apocalypse highly insignificant in the grand scheme of things. However, unlike his siblings, Death is shown to have a more civil and calm manner when dealing with the Winchesters; Death tells Dean that most people also talk to him with respect. Instead of attacking Dean during their encounter, he invites the Winchester to join him at the table to discuss the Apocalypse and even goes so far as to share his pizza with him. Death also claims to be an acquaintance of God Himself, claiming that the two both share a similar advanced age to one another and that neither of them can remember which is older anymore or even if they are the same age.
Despite being callous at times, Death has a respect for the natural order of the universe. He explains to Dean that there are times when it is difficult, but in the end, breaking the order in even the smallest way can cause untold chaos.
While he does not hate Dean and Sam, Death feels that the pair are an affront to the natural order due to their constant resurrections. Even though he is capable of it, as a rule, Death himself does not normally resurrect people (most likely because it disrupts the natural order), though he can and might make exceptions or even "an exception once, not twice." He also seems to genuinely like, to a certain degree, Dean as the two share an almost father-son bond; Death even went as far as loaning Dean his Ring so he can become Death for a day, as a way to teach Dean quality of the Human Soul, along with the consequences of disrupting the natural order. He also appears to Dean when he briefly dies to contact him, something he does on his own as Tessa is both unwilling and unable to summon him. While Dean does lose their wager, Death is satisfied enough by the fact that he has learned his lesson to do as Dean asks anyway, though he says it is also so they will keep digging into what is going on with the souls of Purgatory as he presumably doesn't like the idea of souls being used for power. Unlike the other Horsemen, Death has a level of respect for humans, specifically the human soul. Death tells Dean that the soul is stronger than anyone knows, and that it can suffer much and be broken but can't be destroyed, not even by him.
Over time, Death expressed more respect for the Winchesters, and has even indicated that he was honored to be reaping Sam himself. However, he showed some anger at Sam avoiding Death reaping him as he told Sam that was part of the reason he wanted Dean to kill him. Billie the Reaper states that Death actually found some amusement in Sam and Dean's repeated returns from dying, indicating that was part of the reason he allowed it, but that the only rule now is "What lives, dies."
Despite claiming to be totally indifferent to the Winchesters and Earth, Death has helped on a number of occasions. He gave his ring to Dean to stop Lucifer, agreed to get Sam's soul back, and even helped after Castiel broke the binding spell the Winchesters and Bobby had cast on him, albeit claiming that he was only doing so because he found "that little angel arrogant." He also sharply criticizes Dean for not following his earlier hint about souls and tells him bluntly that he'll help, but added, "Don't thank me. Clean up your mess." It is implied that he is not so much uncaring as he is above the petty struggles of the world, disliking being called on to fix things that he thinks should have been taken care of without bothering him. Death also has an appreciation for human food, especially junk food. His liking for Chicago's pizza was so strong he noted it as the reason he spared the city. In the next season, after Dean lost Death's wager, Death showed up at Bobby's house, with bacon dogs, fries, and beer. Later, when Dean summoned and bound him, he brought him pickle chips in an attempt to appease him, which proved fruitless. Nonetheless, after he was unbound, Death ate the food anyways and expressed his enjoyment of them on his way out.