[Thanatos]

Strength: At least 5 to use most rifles w/o power armor.
Perception: At least 8 to get sniper, but get as high as possible.
Endurance: Crap, don't bother.
Charisma: Whatever
Intelligence: 6 for sharpshooter, though it doesn't really help much. Int is always good for skill points but otherwise unncessary.
Agility: 8 needed for sniper. Get 10 if you can, but you can always boost it with Gain Agility, or else Action Boy.
Luck: High as possible, criticals are essential. Sniper also requires a luck roll.

Skills: Small guns is all you need, but you can get the Tag! perk for energy if you want.

Traits: Finesse. Gifted.

Perks: Sniper, More Criticals, Better Criticals, Bonus Rate of Fire, Gain Agility/Perception/Luck, Action Boy, Sharpshooter. Whatever else that is generally good for any character.

NPC: Whatever rocks your boat. Get a few short range/melee characters like Goris or Sulik to keep enemy crosshairs off your back, but it hardly makes a difference who you pick.

Note I didn't take into account bonuses you can get in the game like the free stats you can get from the BOS headquarters in San Fran. This might not be the optimal formula for snipers in FO2 since I only really bother using power characters in Tactics.