To be honest it was never something I gave much thought to. But here's a compilation of some of the the aspects that have come up in the past with a degree of elaboration.
Nameless typically uses a hat of disguise to appear extremely nondescript and forgettable. Unless someone looks into his eyes, as no magic can disguise the insanity lurking within.
Without the hat of disguise or to those capable of
True Seeing, his eyes glow with the characteristic blue glow of a permanent
Arcane Sight. As a result of his Transcendence, his left arm is now a tentacle of an indefinable purple color that normal minds can never quite nail down the exact color of or that appears exactly the same purple twice. Of course random parts of his anatomy also tend to behave in ways that would disturb normal witnesses.
His forehead used to bear the mark of Cyäegha, a stylized eyeball with wavy tentacles for eyelashes. Though only those most versed in the lore of Xoriat would know it's meaning. Cyäegha is one of The Great Old Ones, Nameless's patron and the entity to which Kha'tvan'ga will deliver him when his life span has elapsed.
Since achieving Transcendence (AKA hitting lv 10 in Alienist), the mark has mutated into an Abberant Dragonmark that allows him to assume an Aspect of Cyäegha and when doing so he assumes many of the physical characteristics of his patron. The tentacles now cover his entire forehead, Even when not manifesting the aspect and the eye is an actual eye on a stalk. Some of the Aspect is visible to those with True Seeing, even when he is not manifesting the Aspect.
There is a faint popping sound and then the skin splits in two-inch long sections all over his body. Dozens of eyes push themselves out of the splits, and a couple of seconds later, Nameless’ face and arms – and, beneath his clothing, the rest of his body – is covered with the orbs, each rolling back and forth as it peers in a different direction. Each eye is a facsimile of the one on his forehead.
His skin tends to have a greenish tinge.
He wears no set style or color of clothing. Instead, picking them to satisfy some arcane and obscure set of criteria (nobody has ever stayed sane through one of his explanations) which might sometime result in fashion disasters or fashion trends, if any ever saw them. Few clothes tend to survive very long in his line of work anyway.
Like all of the Angels, he is covered with a fine network of scars and strange modifications as a result of Mordain Fleshweaver's experimentations and augmentations.