Ugh. First things first, you're going to have to compartmentalize this issue, because it is unlikely that you'll find a lawyer willing to give you any advice that is relevant in HK, the Philippines, and the US. I would focus on the US, unless you have specific need to travel to HK and the Philippines regularly. If you do have to travel to those locations, you should check your legal status in your destination country
before traveling there. If you don't respond to a lawsuit, the plaintiff may win by default, and you may find yourself entangled in a foreign legal system upon arrival.
With regard to the US, you still need to talk to a lawyer. In general, any agreement is invalid without "consideration". That is, you can't enter in to a one-sided agreement, and you can't enter in to an agreement where the consideration occurred in the past. So even if you did sign a document right now, you'd be owed some consideration in order for it to be valid. There are, of course, exceptions to this rule, which is why you really should spend the money to sit down with an attorney who practices business law and is within the jurisdiction in which you are being threatened (California).
It seems extremely unlikely that the plaintiff would be successful in this case, but I have some general advice for you. There's a saying in the US that, "You may beat the rap but you can't beat the ride." This saying means that (in a criminal context) the charges against you may not hold up in court, but you're still going for a ride in the back of a police car if you thumb your nose at authority. I really wish there were an equivalent for matters of civil law, because even a laughably bad lawsuit can cost significant sums of money to obtain a dismissal, or even worse, summary judgement.
I am on year 5 of litigation involving an agreement that A) I never signed, B) never saw, and C) was never a member of the company for which the agreement was made. I was dragged in to the suit on "civil conspiracy" charges, alledging that I knew of the agreement and conspired to assist the signor in breaking it. It has cost me a significant chunk of what would have otherwise been retirement savings. Everyone who sees the case (lawyer or otherwise) says that it's ridiculous, but that doesn't seem to stop the process from dragging on and on.
My advice is this: whatever you may be owed in this situation, weigh it against the fact that it sounds like you were involved with a litigious asshole. Don't mire yourself down in the legal mud unless you absolutely have to. I'm not saying you should acquiesce to his demands, but don't be cavalier about it either. Go talk to an attorney. Ask them to be brutally honest with you, and ask them all the "what ifs" you can think of. Evaluate what risks lie on the other side of a possible lawsuit, even if it is a frivolous one.