Dionysus was often called the 'twice born' because of the two myths around his birth. The most common is that his mother was a mortal woman named Semele, who was a daughter of the King of Thebes, and his father was Zeus. This makes him the only Olympian to have a mortal parent, and according to some traditions the only one to have been born a mortal.
As with most of Zeus' acts of infidelity, Hera became jealous when she found out that Semele was pregnant with Zeus' child. She disguised herself as a nurse (or old crone in some stories) and made Semele begin to doubt that it was truly Zeus. While in disguise; Hera convinced Semele to ask Zeus to reveal his true form and, to make sure that he would do it, to swear on the River Styx, which is sacred to the Greek Gods and an unbreakable oath. Zeus tried to get around the promise, but in the end swore on the River Styx, and Semele asked to see his true form. Unable to go back on the promise, he did so, and she was burned to ashes. Zeus managed to save the infant, sewing him into his thigh until he was born a few months later as a full grown baby on Mount Pramnos in Ikaria. He was born a demigod, like Hercules and Perseus.
In his early life Zeus sent Dionysus off with Hermes. Again here, there are two versions of the story. In one version, Hermes took Dionysus to King Athamas and his wife Ino, who was Dionysus' Aunt. Hermes had them raise Dionysus as a girl to hide him from Hera's scorn. The second story is that the boy was taken to the rain nymphs of Nysa. They raised him through infancy and childhood, and in reward for their care, Zeus placed them as the Hyades in the stars. When he grew up, he discovered the vine, and extracted the juice. He was struck with madness by Hera at this point, and ended up wandering various parts of the world. Rhea cured him, and taught him her religious rites, and he wandered through Asia teaching about cultivating the vine.
He was very handsome as a young man, eager for challenges. Dionysus later said he was good at nothing in his human life but cultivating wine. The people in his village mocked him, not knowing of his heritage, of his father, or knowing his future, and what he would one day become. When Dionysus was invited late to the Mount Olympus, it was Hestia who graciously offered him her golden chair to prevent any conflict or embarrassment. Hestia was a goddess known for her warmth and kindness. Dionysus then rescued his mother from Hades, and she became a goddess on Mount Olympus, with the new name Thyone, presiding over the frenzy inspired by her son Dionysus. When Theseus abandoned Ariadne sleeping on Naxos, Dionysus found and married her. When she died, he went to the Underworld and rescued her, taking her up to Mount Olympus, where he made her immortal. He was disguised as a mortal on the seashore once, and a few sailors spotted him. They thought he was a prince, and tried to kidnap him and hold him for ransom or to sell into slavery. They tried to tie his hands behind his back, but no ropes would hold him to the pole. Some ideas are that the material they were made of then was similar to the vines, and would refuse to hold him. At this point Dionysus turned into a lion and let a bear loose on board. It killed those it came in contact with, and those who jumped off the ship were turned into dolphins as an act of mercy. The only one to survive was the helmsman Acoetes, who had recognized him as a god, and tried to stop his comrades from the beginning.
Dionysus has curly hair so black, it looks purple. He normally has watery, blue, bloodshot eyes from drinking. He is usually described as wearing tiger-striped (or leopard-spot) Hawaiian shirts and purple running shoes (even though he has never run once in his immortal life). He is described by Percy as a cherub who had grown up in a trailer park. Anyone can tell he's no stranger to wine just by looking at him. When he takes charge of the archers of Apollo, he becomes more formal and mature
Dionysus shows his hatred for heroes, as it stems back to how Theseus abandoned his wife Ariadne, and thus is why he has the fewest shown demigod children among the major gods.
Dionysus stresses the fact that he cares little for most demigods, except his own children. He normally doesn't show any fondness towards anyone. He is careful however not to offend other gods, and never threatens them like he threatens demigods. He even respects the archers quite a bit
Dionysus is seen as a 'good guy' because he enjoys ridding others of their madness. Dionysus has been known to enjoy parties (likely because he his the god of festivals).
However, despite the impression he tries to give, deep down, Dionysus cares about demigods quite a bit, and is willing to send help using his godly powers when one of them is in danger and desperately needs help.