While he was among my least favorite among the Black Eagles, he was a good ally.
He was without a doubt, the most loyal to Lady Edelgard.
So he obviously takes her side when she declares war on the Church of Seiros. He was very happy when my Byleth sided with Edelgard instead of Lady Rhea.
And he basically serves as Lady Edelgard’s most important minister during the war.
In battle he was among the most powerful magic users in pure offense. I found him very useful most of the time.