Lancelot (Seiba) NP

I have a support Lancelot Saber, but he's still level 60 since Medals are hard to farm. You can add me at 541,111,221 (Ekenwars).

To answer your question, the damage increase is flat damage from receiving attacks, sort of like Divinity's damage bonus. I believe debuffs such as residual damage have a different system and thus are unaffected.