Perseus
In order to defeat the Gorgon Medusa, Perseus needed to become invisible. To do that, he borrowed Hades helmet of invisibility. In some stories, Athena asks Hades for his helmet. In other cases, Hades appears and offers it. The end result is the same.
Castor and Pollux (Polydeuces)
also the Gemini and the Discouri
Pollux was the son of Zeus and Leda, Castor of Leda and her husband. Pollux was a great boxer, Castor a great horse trainer. When they fell together in battle, they were separated for the first time in their lives as Pollux, being a God, was brought to Olympus, and Castor, being a mortal, was sent to Hades. In order to be always together, Pollux gave up half of his immortality so that they both spent half their time on Olympus and half their time in Hades. The constellation Gemini is named for them.
Adonis
Adonis was a man so beautiful that Aphrodite fell in love with him as a child. So, to keep him safe, she sent him in a golden box to Persephone. Persephone opened the box, and decided that Adonis was pretty cute, also. So, when Aphrodite demanded him back, Persephone refused to give up Adonis. Finally the Gods decided that Aphrodite got Adonis 1/3 of the year, Persephone got him for another 1/3, and Adonis got to do whatever he wanted to for the last 1/3. But, being a good hero, he dies very soon.
Either: Aphrodite uses her magical girdle to make certain that Adonis' own 1/3 of the year is always spent with her, making Persephone so furious that she persuades Ares to kill him in the form of a boar.
Or: Ares grows jealous of Aphrodite spending all of her time with Adonis, changes into a boar, and kills Adonis while he is hunting.
Adonis now vanishes to Persephone's realm all the time, and his blood stains anenomies red.
(Note: Neither Hephastus nor Hades seem to ever do anything about Adonis. Why? Technically they are married to Aphrodite and Persephone, respectively. Very odd.)