Compared
to the other yami / hikari's in the series (Yugi and Yami - Ryou and Bakura),
Yami Marik is trouble-free to tell apart from Malik. For one (if you want
to be completely obvious ^_^) his hair is spiked and gravity-defying. His
bangs also have a more ragged appearance next to Maliks.
On his forehead, he continually has a glowing, golden Eye
of Horus. His eyes are much narrower, harder and callous than his lights,
and he has no pupils in his eyes. When 'normal', we only ever see half of
his iris at the top section 
of
his eye. When he's excited, woundup or shocked, his iris' completely contract,
and become a modest circle in the center of his eye. During later episodes
with him in, his eyes become rounder with madness, and display tiny crimson
veins pumping around. His face, neck, arms and hands are covered in throbbing
veins, and he has a tendency to jut his tongue out of his mouth in a splurge
of insanity. (I must say, he doesn't sound very appealing, ne?!)
Although
he wears Malik's jewellery, he has a different outfit on (I'm not particularly
sure where he got a change of clothing from on a floating ship in the sky...).
He appears to be somewhat more muscular than Malik. I'm not definite if
he's any taller than Malik, as he's usually portrayed to be in fanfiction,
although, Yami no Yugi is taller than his light.
His movements tend to be spasmodic and he’s no doubt a bit of a masochist.
He's undoubtedly very sadistic, enjoying seeing others squirm in fear and
pain because of himself. Put it like this - if you thought Malik was bad,
you ain't seen nothing 'til you've met this guy (yep, he’s the best
^_^)
