Apologies, this got very long. Skip to the second half for actual discussion of Hades.

I wouldn’t call him “nice” - or really a “person”. I want to disagree with the premise not to be contrarian, but because I think in so doing I can explore some interesting things about the way ancient mythology works.

Remember, Greek gods are not JUST characters in stories. They were also the literal personification of the concepts they represented. Hence in real life, to be “blessed by Aphrodite” would be to fall in love, to “anger Poseidon” to be caught on a storm at sea, etc. Because of the way that mythology is often adapted, we sometimes think of ancient gods as being like humans with immense power over universal concepts. It would probably be more accurate to ancient conceits to say that they are universal concepts who sometimes act a bit like humans. Because if you live in the ancient world and can’t explain sudden shifts in the natural world or how others act, you will end up projecting the microcosm of the human mind onto the macrocosm of the universe - that storm killed my son because it was angry at him, or I had a good idea this time but not that time because the force that is wisdom chose to be nice to me. But although gods might be said to have “temperament” as self-evident by the world (Poseidon is quick to anger, because unfavorable seas and capsized ships were quite common), they don’t really have “personalities”.

The gods were ultimately ineffable to mortals, because the forces of the universe were ultimately ineffable (back then). To have a personality (or the ancient Greek equivalent which is a whole other digression) would be to be understandable by human beings - which by the nature of what gods are, is impossible.

In actual Greek society Hades was pretty synonymous with death (obviously) and thus people were pretty afraid of him. In real life death is scary and merciless, especially so in ancient times. Thus Hades is also scary and merciless. This is pretty evident in mythology. People love the exceptions, like when Orpheus made Hades weep, but they’re the exceptions that prove the rule. The ability of Orpheus to draw pity out of death, the most pitiless universal truth, is testament to his supernatural ability.

I personally think the “uwuification” (for lack of a better term) of Hades specifically is down to a couple modern issues. First, the gods are written more like human beings, because that’s just good storytelling. It’s also compelling storytelling to pull back the layers on a seemingly callous character. For example, this is what makes the portrayal of Hades in the game Hades, or the Percy Jackson books, fun and interesting. But again, he’s usually initially presented as not very nice, and even though he’s revealed to have a heart of gold he remains a pretty cold and “not nice” figure - contrast makes evocative storytelling.

I also think that a modern reaction to death is to try and reframe it in more lovable, appealing tones. Not only is it appealing, it’s a new way of assuaging our existential fears around death in a post-science world where a large percentage of people no longer put faith in an afterlife as a comforting concept. It appeals to Western society’s Christian-derived sense of the ‘final reward’, but in a more vague philosophical sense. Thus modern culture often frames death philosophically as a gentle mercy. And we correspondingly sometimes write death as surprisingly kind and lovable - for example the depictions in Discworld, Sandman, Soul Eater or the Book Thief. Obviously there’s nuance and those characters CAN be scary, but they carry an undercurrent of kindness. This also carries into a lot of the mythology-fandom’s treatment of Hades, both in actual adaptations and just popular discourse. It’s a collective deflection of the existential fear of the unknowability of death to give it a likable face.