I’ve seen the Aid-Another build posted in a number of places. This is my take on it.
The goal of the build is to be a tank by making it futile to attack your allies. The core of the build is based around using the bodyguard feat in conjunction with the helpful trait to give your allies +4 or more AC when they are beside you. The best part of the build is that it is extraordinary flexible and can be stacked on top of many different chassis.
For my sample, I will use the Eldritch Guardian fighter archetype to cover this build’s main weakness: its own AC.
Tanky Thomas, Level 1 Human Eldritch Guardian
(20 point buy)
Strength 17 (+2 racial)
With Scale mail and a heavy shield Thomas has a 20 AC, which is respectable. With his familiar protecting him, he has 22 ac. Three times per round, Thomas can attempt to deflect an attack against an ally by aiding another and granting them +4AC (+6 AC for people who are “like family”). Most importantly, this does not take any of Thomas’s actions for the round, so he can happily attack with his longsword for 1d8+3, or use aid another to give a beefier ally +4 to hit.
At the beginning of his adventure with new comrades he probably does not get a benefit from his kin guardian trait, but after he saves the lives of his party members a few times they will become like family.
A quick note: if you read bodyguard strictly according to the written text, you need to be able to reach the attacker and be adjacent to your ally. However, the creator of the feat has said that you should not need to threaten the attacker. If your GM agrees with this interpretation, take a mauler familiar instead, as at level 2, you will share your combat reflexes and bodyguard feat and your familiar will be able to guard you just like a protector, but will have the ability to mix it up in combat eventually.
As he levels, Thomas becomes even more versatile:
Level three is where the trickery happens. We take a level of Beast-Bonded witch and use the Transfer Feats ability to teach our familiar the Helpful and Kin-Guardian traits. This means that our little armadillo can add +6 AC to Thomas or a close ally while aiding another. The circle is now closed and everyone in your party has +6 AC. But wait! There is more! You get a hex too! You can take Fortune, so that you can buff allies when attacking is inconvenient. The best part, is that if you took a mauler familiar or a small familiar, you can now have it aid allies on attacks and grant them a +4 to hit.
At this point, enemy attack rolls are getting high, and they are making many attacks, maybe your attacks of opportunity cant keep up? Enter Harrying Partners! Now you can spend one attack of opportunity to grant allies AC for a whole turn. But how much AC are we granting? At this point, Thomas can afford to buy +2 Benevolent Armor. Ideally, his familiar would also wear the same type of armor. This makes Thomas’ aid-another add +8 AC.
Two more levels of honor-guard cavalier will net you another +1 to your aid another bonus and let you retrain your level 1 bodyguard feat into something else. For your next feat, you could take Swift Aid if you don’t mind figuring out how it interacts with your huge aid-another bonus. A strict reading would give you the full bonus, as helpful specifically sets your bonus. If I were GMing i would just say that swift aid lets you apply half your bonus.
This is the core of my aid build. To build on it, you should also look into some of the cheaper aid-another magic items. The true-love locket will give you another +1 to the AC and attack bonus you grant. At higher levels, a ring of tactical precision will let an ally buy harrying partner or intercept charge for 11,000gp. An alternative interpretation of the build might use Covering Fire to aid another with a bow. That way, you could stand safely behind your allies and aid from there.