He can be parried, but he will only take counter damage for you cannot riposte him.

Can you Parry Lothric Knight Greatsword?

Can ultra greatswords be parried? ... That ultra greatswords CANNOT be parried. So, since a lot of my recent invasions have failed due to parrying, I thought I'd try one out (lothric knight greatsword +10). I am two handing it, and nope.

Can Lorian be backstabbed?

Lorian can be parried in both phases of the fight, although he cannot be riposted afterward. However, he can be poise-broken with continuous hits, allowing to perform a riposte on him.