Vain is an enigmatic being created by the Ur-viles and granted to Covenant by the ghost of Foamfollower in The Wounded Land. His name, which can denote either 'ostentatious' or 'having no real value', further complicates the guessing-game over his purpose. Indeed, what is he? A secret weapon to compensate for Covenant's inefficacy in battle and affairs of love? A hat stand for the Giants? A fashionable vase (place flowers into any moist orifice)? All will (eventually) be revealed...
According to all technical rules, Vain belongs to the race of "made" ur-critters. His skin, hair, and even teeth are darker than Norwegian black metal trapped inside a clump of solid onyx. He is molded to achieve the utmost perfection: Sculpted limbs, and a grin worth advertising black toothpaste grace this being also in possession of a proper nose instead of snot-oozing, gaping nostrils. Perhaps the Ur-viles did not wish him to leave trails of sticky mucus everywhere. Akin to 99.9% of the Land's population, he wears an unremarkable tunic.
While on the subject of inappropriate clothing, it is worth mentioning that only the Giants have been documented to wear pants of any description: While the Land-dwellers and haruchai may continue enjoying a healthy breeze around their privates, they might have become fed up with inadvertent peeks beneath the habiliments of their huger friends, thus crafting a law that anyone stomping around with their privates at the common eye level ought to cover themselves in a proper fashion.
Vain embodies the quintessence of what it is to be enigmatic. To this effect, he very rarely stirs himself into action - a bit like Linden really - and when he does, it's only for his own mysterious purposes. Vain is completely unaware of pain and is almost immune to any physical damage, save that caused by the most powerful of theurgies. He is the only beiong of any sort known to have escaped the clutches of the Elohim, having managed to bust himself out of Elemesnedene.
The Ultimate Purpose
Without giving too much away, Vain has a touch of kleptomania together with a hankering for makeshift jewellery - perhaps because his form is so irredeemably monochromatically black and he's therefore looking for a few fashion accents to soften lines. To this end, he steals the heels of the old Staff of Law and wears them, one as a fetching bracelet and one as a cute anklet.
A certain portion of (female) fans obsess over Vain, transforming him into the Legolas of the Covenant universe: the sleek, handsome bishounen with perfect teeth and strong hands. What does he hide beneath the hem of that teasing tunic? Is he competent in other staff-related matters? Does he respond to other, secret words of command that for instance Linden might have discovered in her maudlin loneliness aboard Starfare's Gem?
As often becomes manifest in fandom, FanFiction.Net sports an entire section of stories about various Mary Sues plummeting down into the Land and falling in hawt lurve with the Prince of the Ur-viles. From this it's clear that some people need to get out into the open air more and take bracing country walks, in order to give their febrile imaginations a break.
The official fan club of Vain, called The Black Staff of Love (Den Svarta Kärleksstaven), is located in Sweden.