He lives in a moving castle, powered by a fire demon named Calcifer. He has an apprentice named Michael. He has been cursed by the Witch of the Waste whom he jilted. He was an apprentice to Mrs. Pentstemmon. He has one older sister, Megan, who lives in Wales. He has a niece, Mari and a nephew, Neil by his sister.
Howl is vain about his appearance, forever wearing gaudy clothes and spending hours in the bathroom. He dyes his hair different colors, as well - its natural hue is a kind of mud-brown. For much of Howl's Moving Castle he wears a grey and scarlet suit which has a charm on it. (To be fair, it wasn't him who put the charm on it - but then, there's no sign he ever tried to get it removed.) Howl is described as "a slitherer outer", and also a "unprincipled, slippery rogue with a glib tongue and a clever mind", both of which are very true. He is a spendthrift. He can be brave only when pretending he is a coward. He constantly falls in love with young women, but only until they fall in love with him, and then he is bored with them and has nothing to do with them.
Despite all his faults, however, Howl is not as bad as he seems. He undercharges customers who haven't the means to pay for his spells and he took Michael in off the street after Michael's parents died, and has looked after him ever since.
Howl turns 10 000 days old when the Witch's curse catches up with him, making him 27 years old. This also places his birthday (calculating backwards from the average date of midsummer) sometime around January 27th.
See also Howl's Moving Castle (spoilers)