This might have been asked but after a search I couldn't find it. Does Imrahil's sphere have to match the ally you put into play using his ability or just the trait? Thanks

Just the trait, so you can pick a card that has any of the Gondor, Noble, or Warrior traits.

The card has no mention of Sphere, and sphere isn't considered a trait.

no need of sphere matching. It just nave to share a trait with him