How would you build an attack of opportunity based character?

[Darg]

Whirlwind Attack is a modified full attack action. Ki Whirlwind allows you to do it as a standard action instead of a full round action. It's still a modified full attack action, not a modified attack action.

Whirlwind Attack exchanges attacks in a full attack action. It isn't a full attack action. It isn’t even an action. Nothing in its description states it replaces or is an action. Consistency requires Ki Whirlwind Attack allow you to exchange attacks. If as you say Ki Whirlwind Attack is simply a standard action, it would do nothing as you don't have attacks to give up.

When you use the full attack action, you can give up your regular attacks and instead make one melee attack at your full base attack bonus against each opponent within reach.

With Ki Whirlwind that becomes:

When you use "a standard action," you can give up your regular attacks and instead make one melee attack at your full base attack bonus against each opponent within reach.

The quotation is verbatim of Ki Whirlwind. Or you can use the attack action. Full attack is to full-round as attack is to standard.