Despite what my lefty friend said (sorry), I think it's much more useful and simple to rule that a Defender is allowed to change allies, and this process takes a month of close contact and study with the prospective new ally. This clears up the thorny questions of resurrection or tricks that Lysander brought up, because a Defender PC would then have the option of keeping the new ally or spending a month re-learning the old one. This also makes the Defender useful as a bodyguard for temporary escort missions, a functionality he wouldn't otherwise have. This doesn't mean he's ever required to change, and the personal devotion in this particular character can be just fluff. Thanks Lysander! Any opinions balance-wise? Right now I'm worried that Devastating Blow isn't strong enough with 3d6 and every-other progression, but I'm not an expert on the sort of damage expected at higher levels.

I'll edit the OP to add the ability to change his ally.