Hermes gives boons that increase your mobility, attack speed, evasion, dash capabilities and cast recovery.
If given Nectar, Hermes will give you the Lambent Plume.
Hermes' affinity gauge is maxed out at 8 hearts.
Hermes' favor requires speaking with him while having the Lambent Plume equipped and max leveled to 3*. It does not matter how many stacks of its bonus you have.