Rikku because she always get attacked by Shinryu .

My Rikku have low HP about 8000+ like that while others have reached 9999, my Wakka and Tidus reached 20000+ with Break HP Limit. My kimahri is slow but he counter the enemy and strike the enemy back with Evade & Counter. My Lulu is always good at dodging and it's funny to see bigger enemy can't hit Lulu while Lulu can done 25000+ with a normal physical attack .

Actually Kimahri can be one of the strongest if you send him to Tidus's, Wakka's or Auron's path. His HP, strength and defense will be higher if you send him to Auron's. Strength, Agility and Evasion can be obtained highly by walking to Tidus path, all moderate stat can be obtained from Wakka's path because Wakka is a balanced character.