There are several ways to optimize this build as far as I can see:
1) Not enough healing: As far as I can see, your only ways of healing are Champion's and Cait's self heal and items, which doesn't seem too reliable. As far as im concerned, Raiment Ryn is the best healer in the game, but if you want to keep Cait for RP reasons, switch her to the Witch outfit. Also, you could switch Mirror Stance with the new Lay on Hands (also helps with aggro but allows you to offheal), although it is not a necessity as long as you get a dedicated healer.
2)Your stats are not properly optimized for any role. It seems to me like you are your party tank, so I would advice to max Strength, Agility and Toughness (you can boost either Presence or Cunning with drinks). If you are your party dps, max Strength, Agility and Cunning (boosting either Toughtness or Presence with drinks).
3)Regarding your items, there are some suboptimal choices and anti-synergies:
- Rogue Ring is nullified by the Helm of Heroes, given how the Helm is a better item for your class and role, you should replace the ring with one of these alternatives: Mare's Ring, Lucky Strike or Charm.
- Senninbari is only good if you are getting your ass kicked, and shouldn't be equipped unless you expect that (especially since Cait already gives you a safety net with Devotion). Replace it with a higher stated item and equip the Senninbari reactively in the middle of a fight (changing items does not take up your turn after all).
- The War Scythe is meant for spellslinging. If you want to use a Two Handed Melee Weapon, try Galon's Griefmaker or the Weeb Sword.
4) Mark for Death is only good if you are not playing in manual mode. I would recommend switching it with Cleave given how you are lacking in AoE. Another alternative would be Grease (to support Brienne and CC on demand) and Lightning Fist (stun on demand and usable while unarmed). I can't make any comment on Steady Strike tho, I haven't used it but I can't find any flaws on doing so.
5)If you don't want to tank but rather be your team damage dealer, a nice movepool would be Sneak Attack/Shadow Strike/Cleave/Deadly Shadow. The main idea is to open with Deadly Shadow and follow it up with either Cleave or Sneak Attack, and the keep using Cleave or Shadow Strike followed by Sneak Attack (which is the melee equivalent of a Greaseball as far as I can tell). Regarding this set, a Thief would be able to net more damage but that would requiere further setup to exploit Twist the Knife.
6)Regarding you Ultimate, most ultimates suck ass, but if you are playing a Kitsune you always have the option of using Kitsunenuki (you unlock it by equiping a Fox Jewel, but do not need the jewel permanently equipped to use it).
Finally, regarding the Royal Leathers, it is not a really good item but I'm assuming it is there for RP reasons, so I'm not making any further comments on it. Hope any of these tips are useful.