Fodder off neutral or +Res/-Def Fallen Robin (M)?

+Res -Def needs to go, he needs all the Def he can get

Depends what you're using him for. For general usage I would go +res -def due to balanced bulk (good with steady breath + distant def, for instance), but if his main role is to handle non-manakete melee units and he doesn't mess with mages much except to OHKO blue mages back, neutral is better.

For being as slow as he is, I think neutral is probably the better choice for most people. He would really need to invest quite a bit into his res to be able to take two hits from a non-blue mage, and Fae does that more easily. Meanwhile, his ability to handle two hits from melee units is going to suffer, at which point you're practically building him to be a res tank, and I would say Fae is better for that. To go the route I just described, you would need some special justification, like, melee threats are very well handled on the team already and DC Effie already brutalizes any red mage. Something like that.