Arwen Undómiel (Spirit) says:

Response: After Arwen Undomiel exhausts, choose a character. That character gains sentinel and +1 [Defense] until the end of the round.

Imagine that you choose to exhaust Arwen to deffend an enemy and trigger the response.

Can Arwen be the choosen character to get the defense bónus?

i would say that as long as you arent using the sentinel to defend another deck's enemy (as she could not have been declared before she exhausted) then the +1 defense is fine, as she has exhausted and got the +1 before damage calculation is caried out

rich

Of course that she cannot be deffending another player's enemy...

Should have written in a solo play.

Thank you rich

Yes, I often use her response effect on herself. She can then withstand at least one hit from an enemy with 3ATK (of which there are many in later scenarios, such as HoN). All-in-all, a really solid ally.