Welcome to my second proper guide. I am writing this when bored and tired, I also have a slight headache, so any spelling mistakes I make I would appreciate being pointed out and I should change them as soon as I see your comment.

However, im here to talk about Karma, so may I say that Karma is a very effective tanky melee support. She works incredibly well with champions who jump into a fight and chase down the enemy, champions such as Jax, Jarvan, Xin Zhao and Malphite. This is because her shield has a high AP ratio and is as close to a heal nuke as she has, and absorbs are done post damage reduction, so high armor/MR champions synergise well with her. To compare her with another shield supporter - Janna - she performs very well with champions who are more melee based and tanky, whereas Janna is much more suited for supporting ranged champions who are more squishy, this is shown by Janna having slows and CC to keep away enemies, contrasting with Karma who has abilities which benefit her being close and involved with a fight, and helping those who do that also. Compared to supports in general, her heal is weak unless used properly with the correct timing, her abilities require thought when used before hand, as she essentially uses two cooldowns at once on some abilities.

However moving onwards, that shall be it for this chapter, next, I will briefly discuss my rune and mastery choices.

My runes are my basic AP runeset. You may take flat regen seals instead of /level, and you may take ap/level glyphs instead of MR (useful due to huge AP defecit in this build, will discuss later).
My masteries are taken to boost my ghost and clarity summoner spells. The rest is fairly generic as far as supports go.
I tookas it is useful for keeping up with and catching up with teammates and enemies alike. It adds a lot of survivability to an already survivable character, and provides a movement boost for escape for when you cant mantra spirit bond away from a fight.is an amazing summoner spell for many reasons:
1- Can restore mana to nearby allies, acting as a 'mana heal' if you wish. this allows you extra staying power in continuous fights, primarily when poking where attrition warfare is the name of the game.
2- Gives staying power in a lane which helps early game gold generation and harrasment
3- In midgame conflicts when mana can be an issue, it has often saved my life when I didnt keep track of my mana when getting carried away in a fight. Stupid, but happens to all of us, also, sometimes just one more spell with that little bit more mana can get that kill, save that ally, save your life.

I feel thatis the expendable spell in this case, good replacements are clairvoyance, perhaps exhaust or rally. Bad choices would be smite, heal, revive.

This build is meant to be tanky support. The item path I gave was for a team of which consists of mainly physical attackers with some CC.
I will go through builds for different team comp, however your role will stay the same throughout.

Heavy CC enemy team, Magic based.

->->->->->
This build is focused on heavy MR and CC redux. The weaknesses here are that you have low mana, so golem buff is an important thing to get, as well as careful mana conservation. If mana problems become apparent after mercury treads, getting a tear of the godess would be advised to counter these problems.

Heavy CC team, Melee based.

Same build as in the description, butfor reduced CC duration.

Mild CC team, Magic based.

->->->->->

Mild CC team, Melee based.

Same build in the description.

Almost no enemy team CC

(pseudo CC on enemy team such as immobilises)
->->->->->
OR in case of:

Heavy melee damage
->->->->->(may also use the build for mild CC melee based)
Heavy magic damage
->->->->->
This should cover almost all enemy team compositions. Mix and match between item paths when needed: for example, more than mild CC, but not enough to warrant going full counter-cc, you could switch boots. Need more armor or MR, swap some items. This is just a framework on which you can follow and build even further from, as most builds are, dont follow this exactly if you feel you need a specific item vs a certain team composition for example.

Moving on to playstyle.

Karma playstyle, especially with this build, is that of a tanky support whom is meant to be close to the fighting, helping deal damage while aiding her allies. All of her abilities, much like her name, fit into the theme of balance.
Her primary damage ability is also her heal. Her movement speed buff also slows and damages, her shield also weakens the surrounding enemies.

She gets these additional effects (in the case of her movement speed modifier [spirit bond] it just increases the movement speed effects, rather than adding an additional effect) by activating her 4th ability (availiable from first level, and has no ranks): Mantra.
Because she has no ultimate, her 3 abilites go up to rank 6. This is one of the reasons why I feel Karma does not need as much ability power as other supports may need: higher ranks = higher base damage / healing / shielding. The other reason is she gets AP from her passive has her health is lowered.

A description of her skills

Her first ability is Heavenly Wave.

This is a short range conal AoE attack that deals moderate damage.
With mantra, it heals for a fixed amount plus a percentage of thier missing health to both Karma and all allies in the cone of effect.

Since this scales off of missing health primarily, low hp targets with high amounts of maximum HP will be healed a lot by this. This is one of her benefits in teaming with tanky DPS characters.

Her second ability is Spirit Bond.

This is a tether that acts in different ways.

The tethers target is called the Anchor. If the anchor gets out of range the spell is broken prematurely. Minions, and some placable objects such as Jarvans's Flag (if i remember rightly that is) can be used as anchors. The anchor will recieve a movement speed modifier, either an increase or a decrease corresponding to friend or foe.

Karma also recieves the movement speed increase. Any ally who moves through the beam between Karma and the anchor will recieve a movement speed increase. Any enemy who moves through the beam will be damaged and recieve a movement speed decrease.

With mantra the movement speed modifiers are doubled.

Her final ability is her shield, Soul Shield.

The target gets shielded. It can target minions (which you should do when pushing and you want minions to tank more turret hits, also to be used when harrasing, giving a rather large range of damage).

As with all shields, the absorb is done after damage reduction, so targetting tanky characters will make the most out of the shield.

With mantra it deals a burst AoE damage around the shielded target for the damage as the shield shields for. This is very nice for initiators who jump into a fight, as it straight off increases thier health and reduces the enemies health, hopefully scaring them away, as running means they arent attacking >:3.

About Karma Herself

As you can see, tanky melee characters are the best characters that Karma synergises with. Personally, Jarvan is the perfect compliment to her. Jarvan also has a shield which does an AoE slow, he has crowd control and jumps into grounds of enemies, and so lets you deal high burst damage with your shield. For similar reasons, Malphite is also a good partner to Karma. For lesser reasons (the jump and being slightly tanky) Jax is also an ok partner for Karma.

Karmas shield is her best early game damage option. I tried different spells in the beginning, but the shield turned out to be the best choice. It helps protect yourself and your laning partner when harrasing, it has high burst damage, on a moderate cooldown (mantra cooldown) which helps limit your spell casting for mana conservation. The AoE burst range is also quite long.

Karma should play quite agressively early game, as since everyone has no items, her lack of AP in her build is a non-issue in this phase. Also, if you can get the enemy team to hate you, your tanky items will help you survive as well as allowing squishier people to remain unharmed.

I leave her tether ability until later in the game, while its damage is high, and the movement speed can be useful, I much prefer the extra healing gained from heavenly wave, and the damage part of spirit bond is hard to reliably use.

Karma is a very strong farmer. She can decimate a minion wave in just mantra shield -> heavenly wave, you should take advantage of this and feed yourself as much as you can (but not to the detriment of your carries) in order to get your self nice and tanky.

Typical Situations:

When starting a teamfight:

Let your initiator such as Jarvan or Malphite rush in and mantra shield him as soon as you can, spirit bond him and catch up and get in close, judge the situation: if his damage intake is low, use a nonmantra heavenly wave for the damage, and if off cooldown, a nonmantra shield. When he gets low use a mantra heavenly wave. If you get focused, pull the enemy back behind the tank and into the group of nukers/carries and then kill them, if its a group of enemies, decide whether to split off from the team and draw away the enemies if you feel your team could not win the fight, or take the damage and try to kite with spirit bond while your team kills them, if you feel confident.

When chasing with a teammate:

Let your teammate chase and do damage, use a mantra spirit bond on your ally if you need speed quickly, or on the enemy so that K--A-E (K for karma, A for ally, E for enemy), this will speed up yourself and your ally, and slow the enemy, almost always ensuring thier swift demise.

When being ganked:

Run away the moment you realise the danger, and preemptively shield yourself, as to leave its cooldown for a second cast if needed. Use a mantra spirit bond, and run away to safety, being either turret or allies. When you get low shield your self again if you have it, and mantra heavenly wave yourself for a nice boost in HP. Hopefully you survived, if not, go flip **** and call your teammates ******s for not calling misses even though you got ganked by a jungler etc etc actually DONT DO THAT. :3.

This will be a brief chapter on why I build Karma in a way that may seem unorthodox to others.

1: Karma is a support that is based on survivability, being close into a fight, and dealing respectable damage for a support, which makes AP builds viable, and the oestensible no-brainer build choice.

2: Karma, as a support, and as a support that is hard to kill, will be targetted. She has high damage for a support, strong shields, and AoE heals, which greatly increse her teams survivability, and targetting her is a very sensible idea.

3: Karma does not rely on items for her abilities as much as other champions might, for example Sona is very mana dependant and so requires items to allow her to use her abilities as much as she can in order to be as useful as she can be, Karma can survive without such items, and so can build survivability without a serious loss in her abilities strength and usefulness.

4: Having a tanky character in your team alongside your tanks improves your overall team survivability. Having yet another squishy AP caster in a team isnt supporting anyone when said caster super fast from being attacked a few times.

5: Its just plain hilarious being a tanky Karma in a kimono (I use the Sakura skin) with everyone raging hard when they cant kill you, then realising your team just killed them. So much hate from enemies; so much love from allies. Just so epic :3.

Pros:
High Survivability
Good Farming
Excellent Harraser
Amazing at supporting melee champions
Cons:
Weak damage compared to AP builds
Potential Mana problems
Not efficient at supporting ranged/magic based teams

Summary:

Karma is a very good support that is tankier than most, and this build seeks to make the most out of this. It does this by treating Karma a a tanky support character and building as such, and playing in a way that is befitting of such a build: being 'in the thick of things'.

She is well suited to champions whom are tanky and initate such as Maplh or Jarvan, but for champions whom arent as tanky and such dont benefit as much from her shielding as others, she will not work as well as she possibly could, and a different support would be more befitting such a team composition.

This build is meant for supporting a team with melee characters, that is missing an offtank besides the main initator/ main tank. In such a team this build will shine. In other teams, this build may be too overkill in defence, or may be too underwhelming in support due to bad synergy with the team composition.

Thank you for reading this, I hope it was fun to read in some way, as it was fun for me to write, and I hope it helped you in playing Karma, or perhaps not playing Karma and picking a better choice for your team. Goodbye, and one last thank you ^_^.

Here I will write answers to questions I have been given, in order to save comment space and in order to give a consise collection of improvements and recognitions to those whom have helped this guide.

Firstly, the question by Trophycase, asking about how well Karma is effective versus other tanky support builds, notably, his own (well written and concise i will also say, Taric build).

To be perfectly honest, I dont know. I believe the best answer would be to look at what the supports offer.

Taric offers a very nice CC/single target burst damage ability, as well as increased armor and some AD increase. I typically play Taric as a tanky support whom excels at pushing turrets. I believe he is much better suited to supporting a team whom has easier to kill people.

I say this for the following reasons: His heal also heals himself, and is effectively on a short cooldown, so you can not feel guilty about using it as much as you can, which is needed on easier to kill allies.
Secondly, his shatter aura gives him and his allies increased armor which due to the exponential decay of the value of armor, is only efficient when with low armor teams, rather than when playing with tanky people.
Thirdly his stun is good for setting up skill shots, as well as buying time for squishy members of your team to escape from those whom seek to kill them.

From this, I would conclude that while Karma and Taric are both good tanky supports, Taric is more befitting to support squishier teams and lane with squishier carries, as shown by high volume heals and armor increases.

Karma is more befitting to a tankier team as shields are much better on high armor targets and her heal is more efficient when healing low health people whom have a lot of health items : tanky people. She also has nice mobility boosts for a team, which could be seen as her pseudo-CC on enemies, though nowhere near as effective as a two second stun.

I hope this was a good answer and helps others also.