Pros
-High damage output for support
-High priority target
-Items help both you and team
-Unexpected
-Good harassment
-Distracting to the other team

Cons
-Little farm
-No sustain
-No extremely high burst damage
-High priority target

Voracity
After a kill or assist, Katarina gains 25 gold and her cooldowns are reduced by 15 seconds.
Great passive. This is definitely going to rake in the dough while you support your lane partner. This combined with Mercenary in the defensive tree is bringing you in 50 gold per assist.
Bouncing Blade
Katarina throws a dagger that hits up to 2 / 3 / 4 / 5 / 6 enemies, dealing 60 / 95 / 130 / 165 / 200 (+0.8 per bonus Attack Damage) (+35% of ability power) magic damage. Each subsequent hit deals 10% less damage. It won't bounce to hit the same enemy again on the same cast.
This will be your harassment tool while waiting in the bush. Use it every single time it's up on the enemy lane or a caster minion so it can bounce to them. Just be careful not to kill the minion in the process.
Killer Instincts
Passive: Katarina's abilities and melee attacks deal 8 / 12 / 16 / 20 / 24 additional damage each hit.

Active: Katarina's next basic ability has a bonus effect.

Bouncing Blade: Deals full damage to every target it hits and applies 50% healing reduction for 5 seconds.

Shunpo: Katarina takes 15 / 20 / 25 / 30 / 35 % reduced damage from enemy attacks for 3 seconds. Also, it causes Katarina to appear behind her target.
Really good ability to activate before you use your Q on the enemy especially when they have a healer since it puts a -50% healing debuff on them. When you harass with Q, try to use this before hand. Also, if you and your lane partner are engaging, use this before you shunpo to take reduced damage.
Shunpo
Katarina moves to her target's location, appearing behind it. If the target is an enemy, she deals 80 / 120 / 160 / 200 / 240 (+75% of ability power) magic damage to the target.
Awesome ability. Gets you in and out of fights like there's no tomorrow. You can shunpo to wards, allies, minions, enemies, pretty much anything. Shunpo does a lot of nuke damage as well even if you're support. Q followed up by a quick Shunpo is great harassment.
Death Lotus
Katarina goes into a state of extreme focus, throwing 30 daggers over 3 seconds at the closest 3 enemy champions. Each enemy champion can be hit by a maximum of 10 daggers. Each dagger deals 50 / 65 / 80 (+0.5 per bonus attack damage) (+25% of ability power) magic damage.
Great damage even with no AP; 500. Plus, it scales with both AP and AD, but not like it matters because you're support Kat. This is also what makes support Kat decent. Everyone in their right mind will go out of their way to blow all of their CC on a Katarina. It's built in the back of their minds to stop you from flinging deadly daggers in their faces. So while they blow all their CC on you(even though you do no damage), your whole team is free to unleash everything on them. Oops.

Okay, before you guys start raging and QQ'ing about everything, hear me out. Support Katarina works! And this guide is going to show you exactly how to do it. If you're looking for super shunpo burst or press R and win awesomeness, get out right now. This is support Kat. Learn it. This is my first guide and I didn't want to make an ordinary guide with ordinary items. I like to mix it up a bit, and when I tried Support Katarina, I actually liked it. She has been my main for a while so I know a bit about her and if you like her too, try this and have some fun.

Greater Mark of Insight
Magic Pen.
Best choice for reds. I don't see any other option here.

Greater Seal of Vitality
Scaling Health.
Good choice for yellows. Since you won't be getting Rylais or anything, I would go with these. Other good choices would be:
Greater Seal of Resilience
Armor
Greater Seal of Endurance
Percent Health(Although I've never tried these out yet since they're relatively new).

Greater Glyph of Celerity
Scaling Cooldown Reduction
Cooldown reduction is always good on Kat. Other good choices:
Greater Glyph of Shielding
Scaling Magic Resist
Greater Glyph of Force
Scaling Ability Power

Greater Quintessence of Potency
Flat AP
These Quints will give you that Oomph in your harassment early game. Other good choices:
Greater Quintessence of Swiftness
Movement Speed
Greater Quintessence of Fortitude
Flat Health

9/21/0
Pretty standard Katarina Masteries except for a few differences here and there.

IMPORTANT MASTERIES
is essential for more Magic Pen.

for the improved Heal.

Obviously.

for the extra health to keep you alive.

The reason why I go 9/21/0 and not 9/0/21 or 0/0/30 is because even though you are supporting, you still need defensive masteries to keep you alive. It's up to you and your play-style.

Pick upand a to start off. Keep either the side bush or dragon warded for ganks.

Try to stay in lane until you have enough gold for a and a If you end up going back early, at least pick up the Kages and make sure to buy wards and health pots as needed!

Once you get the two gold per 5 items, you'll start gaining a lot of gold.

You want to buy a as soon as you can to start helping out your team. Although, if you do not have many AP on your team or your AD are doing extremely well, I would pick up a as soon as I can.

It should be around mid game and you will start getting more AP while helping out your team at the same time. At this point, you can choose between two different items.

If the enemy team is stacking health and you just can't seem to kill them, pick up a and make sure to use it first to take a huge chunk of their health out.

If you and your team are doing well but the enemy team is starting to get some MR, get and let them know that you mean business. AP, and an aura that reduces the enemy teams MR. Your damage and your team will love you.

If the game is still going on and after you pick up both of those, reward yourself with a nice and you're good to go.

Make sure to grab wards and Oracles as needed, and even elixirs for that extra damage.

I like to get 3 ranks of eachand while getting at least 1 rank of early on. After you've gotten both Bouncing Blade and Shunpo to rank 3, start maxing out Shunpo. Obviously grab your ult at 6, 11, and 16.

HealClairvoyance

or
HealExhaust

or
FlashHeal

You always want to grab Heal since you don't have any sustain for your lane partner. Also helps your team during team fights. Good spell for baiting kills as well.

Well, that's support Katarina for you. Something different, something new. Try it out and see if you like it. Don't expect amazing damage or anything because it's not your normal Katarina build. Do expect a decent lane partner, a good distraction for the enemy team, and a nice big pair of bouncy..........................................................................blades.