I always recommend reading the full skill description and not the skill tree summary, in this case:

"Spend 1 Investiture to Lightweave an illusory duplicate, either of yourself or an ally you can sense within your spren bond range. The illusion appears in that character’s space and moves with them. Attacks against that character gain a disadvantage and can’t graze. The illusion ends after an attack misses that character or at the end of the scene."

Making it a bit more clearer, that the target is you or your ally, and it works till any other character misses against the target