Forming a pair along with Knight (2), when in a Party, these cards take on all the damage which would otherwise have been done to other characters.
Along with its quite reasonable Power for the cost, Knight can significantly improve a Party's Power while allowing you to push some weaker cards through comparatively strong Blockers with minimal losses.
He also has enough Power to make Breaking him through direct damage at least take a good effort.
Of course, your Knights still generally won't last long. If you use them well, they'll usually Break in their first fight, protecting their Party from something much stronger.
Knight combines well with other cards which improve Parties, like White Mage (3), Wakka (2) or Chocobo (6). Concordian King and Chocobo (5) can also improve your Party choices. Aqua and Wind have most of the Party card, so combining them can produce some interesting results.
Whether you're aiming for a strong Party deck or just want to some protection for your Aqua Forwards, Knight is definitely a card to consider!