This probably belongs in the "rules" forum, but here's my spin on it...
A knife thrower really needs to be mobile, since knives don't do the damage of bigger weapons in a melee skirmish.
Hence, you want to give the knife thrower the ability to get into tight spots and out of danger - the Tumble skill is a must.
Since knives are thrown weapons and rely on your Dexterity, and Tumble requires you to be lightly armored, it is likely a good idea to put your highest attribute as Dexterity.
Now, you will need to use Feats to help your knife throwing. You positively must have Quick Draw or you'll be wasting many actions pulling knives out. To increase your rate of fire, Rapid Shot is also a must (and therefore so is Point Blank shot, its prerequisite). You'll also want to get Weapon Specialization ASAP to add to your knife damage.
I would suggest:
Human Ftr1:
Max Ranks in Tumble
Dex as highest Attribute
Feats: Point-Blank Shot (Character Level 1), Rapid Shot (Fighter Level 1), Quick Draw (Human)
Level 2: Ftr2
Max Ranks in Tumble
Feat (Fighter Level two): Weapon Focus - Dagger
Level 3: Ftr3
Max Ranks in Tumble
Feat: Far Shot
Level 4: Ftr4
Max Ranks in Tumble
Feat (Fighter Level four): Weapon Specialization (Dagger)
Level 5: Ftr5
Keep the Tumble ranks maxxed
Level 6: Ftr6
Feats (Ftr6 and Char Level 6): Dodge and Expertise (both will really help your defenses; important at these levels where a high AC still matters)
Level 7: Ftr 7
Level 8: Ftr 8
Feat (Ftr 8): Improved Critical (Dagger) - Self-explanatory, I think.
After this point, you've picked up all the stuff you really, absolutely, postively need, Feat-wise, and it may be a good idea to pick up Rogue levels for the Sneak Attack damage and other bonuses:
Level 9: Rog 1
Feat (Char Level 9): Improved Initiative (gets you a few more Sneak Attacks while foes are flat-footed)
Ability: Sneak Attack +1d6
Level 10: Rog 2
Ability: Evasion
Level 11: Rog 3
Abilities: Uncanny Dodge, Sneak Attack +2d6
Level 12: Rog 4
Feat: Mobility (great synergy with Tumble)
Level 13: Rog 5
Ability: Sneak Attack +3d6
Level 14: Rog 6
Ability: Uncanny Dodge (no flanking)
Level 15: Rog 7
Feat: Shot on the Run
Ability: Sneak Attack +4d6
Level 16: Rog 8
Level 17: Rog 9
Ability: Sneak Attack +5d6
Level 18: Rog 10
Feat: Precise Shot
Ability: Opportunist (do Str Damage with every sneak attack

- VERY useful ability)
Levels 19 and 20 are your choice. I can't think of anything in the Core rules that really needs adding to make you much more effective - though perhaps two levels of monk (for both the Wis bonus to AC and Deflect Arrows - and other missiles) would be a good fit.
--The Sigil