What happened:

The Plot card Besieged was played. Theon Greyjoy attacks me.

I have someone to defend that is str 5 - does he kneel and def with 0 str? I am wondering about the "if able", and can a 0 str participate?

Thanks!

Gabe

Theon Greyjoy (TFoA) does not stop characters from participating; he merely prevents their Strength from counting in the challenge. If that 5 Strength character is the only one able to participate, then the challenge will be undefended.

Awesome, thank you. I guess my last question is - does the defender need to kneel? (Because of Besieged) even thought it will count as 0?

Yes, declaring a character as a defender kneels it, regardless of its STR or whether it counts (Theon doesn't change the defending character's strength, he just makes it not count).