Tiefling
* +2 Dexterity, +2 Intelligence, -2 Charisma.
* Medium size.
* Darkvision out to 60 feet.
* Racial Skills: Tieflings have a +2 racial bonus on Bluff and Hide checks.
* Racial Feats: A tiefling gains feats according to its class levels.
* Special Attacks (see above): Darkness.
* Special Qualities (see above): Resistance to cold 5, electricity 5, and fire 5.
Demon
* Immunity to electricity and poison.
* Resistance to acid 10, cold 10, and fire 10.
* Telepathy.
* Damage reduction 5/cold iron or good (or better)
* +5 (or more) Natural Armour
Well, comparing Tieflings and the 'basic' demon shared traits, the tiefling has most of them already.
If you're willing to use the 'Savage Progression' idea from the
WoTC website, you could call it a template class.
Probably a three level 'class' would do the job, something like the below. It's not hugely interesting, but some flavour text should help there. If nothing else, I would think that the character would become more and more identifibly 'demonic' in appearance as the Natural Armour increases. You should probably specify that the character gains an Aura of Evil (regardless of actual alignment) at some point, either at 1st or second level if the PC isn't evil already. The spell resistance isn't hugely useful in some ways (it won't stop someone near the characters own power level very often), but it will help against the mooks: and hey, it might only be a 10% chance of stopping that disintegrate, but that 10% could make all the difference. Alternatively, it could probably be pushed up a bit without doing too much harm to balance.
Level 1:
Resistance to Acid 5, Cold 5, Electricity 20 and fire 5 (totals)
Bonus to save vs. Poison +10
Telepathy (Range: Touch)
+1 Natural Armour
DR 5/Cold Iron or Good
Level 2:
Immunity to Poisons
Resistance to Acid 10, Cold 10, Electricity 30, Fire 10 (totals)
Telepathy Range 100ft
+3 Natural Armour (total)
Level 3
Immunity to Electricity and Poisons
+5 Natural Armour
Spell Resistance ECL +2