We know Pluto (Hades) as the Lord of the Underworld and Riches.
He has a rather bitter personality and I find him rather "pessimistic", seeing the darker side of everything. Nico di Angelo appears to have inherited this personality trait from him.
However, we see Hazel as very cheerful, not so down like Nico is. Does this imply Pluto may have a cheerful side to his personality?
It does suggest that Pluto (and maybe Hades) has a jollier side as most children reflect their parents (Aphrodite's children are vain and/or caring, Athena's are arrogant, etc.). Now in theory Hazel's cheeriness could come from her mother but from what we see of Marie Levesque this seems unlikely. Maybe she was cheerier before the curse but I doubt it. Looking at more evidence for a jolly Pluto I found a little but not much.
Before I go into that we need to remember a few things. We see very little of Pluto in the books. He speaks to Hazel in a flashback and I think we see him again later but mostly who we see is Hades talking to Nico or Percy. This is important as the two are different, Hades is less respectable, he is very much a death god. Pluto on the other hand is a god of riches and wealth as well as death and is seen as moderately more acceptable. This can be seen in the way Hazel is much more accepted by the Romans than Nico by the Greeks. Another big factor is how Hades feels in his appearances. His partner was murdered by Zeus, his children forced into hiding and he is shunned by the Gods. This is on top of all the bad stuff that happened to him in the myths and his appearances in Percy Jackson are always when he feels he has been wronged or is in a stressful situation.
In the books there isn't much evidence for Pluto having a happy side. He tells a joke, maybe two, in Blood of Olympus chapter 14:
Hades allowed himself the slightest smile but there was nothing cruel in his eyes.
This comes straight after him teasing Nico about his obsession with Percy. He also says to Nico:
Prehaps your chambers would look good decorated with the skulls of monks.
And after this his eyes glitter, possibly showing merriment.
He cares for his children and lovers, he gives Hazel some presents and begs Marie not to endanger her as well as telling Nico that he should try to be happy.
I believe but can't find the section that Hades also seemed more cheerful when with the DiAngelos' mother.
I believe the reason for Hades' lack of cheerfulness is summed up here:
You convinced me too let go of my anger
Emphasis mine.
Hades's anger at his unfair treatment is what keeps him so upset. When with his children he seems to occasionally overcome his anger and be happier. On the other hand when Hades is alone his anger intensifies. Nico is also angry and alone which is why he shares Hades' personality. Hazel is usually not alone, being either with Sammy, Frank and Percy or Leo. In Alaska when she was isolated she does become sadder and less cheerful.
I would say Hades/Pluto could be cheerful if he was with people but his exile from his family on Olympus and the 6 months he spends without his wife cause him to become sour and angry, squashing his happier side.
