What should I do to start a "stealth" or "assassin" character in Dark Souls?

Solution 1:

What you do in character creation doesn't really matter. The only difference between starting characters is their equipment (spellcasters have an advantage that can last a while). You'll be able to smooth over the difference in stats very quickly.

For a fast, finesse-centric (not stealth! There are only a few opportunities to sneak up on or past enemies in the game, although there may be some magical aid hidden somewhere) character, get some form of short sword, dagger or rapier/foil for your main hand and a parrying dagger or buckler for off-hand. Basically, give each new weapon you find a few test swings to see which feel best to you. Light weapons tend to scale with Dexterity, so invest in that.

Use light armour and keep your equipment load low. You get a mild penalty to your rolling speed at 25% maximum weight, and a heavy one (that makes rolling A Very Bad Idea) at 50%.

Keep in mind that even though such methods are quick by Dark Souls' standards, the game in general still has relatively slow and methodical combat, so expect to fight defensively, keep your distance and try to parry blows a lot. Also, don't ignore other weapons entirely. A long stick and a heavy shield can make certain areas a great deal easier, even if they aren't your best weapons.

Solution 2:

As mentioned, the most important thing for a stealth build is to keep your equipment load below 25% as this gives you access to the improve dodge, which really does work much better than the typical heavy dodge.

• Get the black "ninja" style armor from Mid-Blighttown, not far from the first bonfire in Blight Town. This is very light and gives you strong resistance to poison, plus you look like a bad dude
• Thief starts with Bandit Knife. This knife has excellent critical but will begin to lose effectiveness in the mid-to-late game even while upgraded. Therefore, you will want to switch to a katana-style sword or another fast weapon sometime before this happens. Most weapons swing remarkably slowly (compare the morning star or zweihander to the dagger and you'll see what I mean.)
• For ranged weaponry, a bow and arrow makes a lot of sense, so grab the Composite Short Bow from the end of New Londo, above the lever
• Fast, stealthy characters tend to be good in PvP but weak against bosses because they do very little damage with critical. However, a +10 Uchijigakatana or Ioiato (also in BlightTown) with Golden Pine Resin actually does a lot of damage with high Dex
• Get the Wood Grain Ring by killing the Ninja that hangs out with Shiva of the East in the Haunted Woods (you'll usually end of killing Shiva in the process, so make sure there's nothing you want to buy from him.) The Wood Grain ring improves your dodge even further and is the most essential thief item