Honestly I have updated this in a bit, but need to. For clarification it is Gellius > Deine > Deina.
Gellius is the best at actually tanking. Which is their job. Both Deina and Deine have offensive skills.
Deina either gives you a flat 100%(1016) increase in damage and allows her to attack at range (200). While her HP is 4069 and her Def is 663. Though she has the ability to essentially paralyze a target 25% of the time. Though if you luck sucks... well...
Deine on the other hand gives you a flat 250%(1841) increase in damage and allows her to deal AoE damage. So she clears out what she is tanky faster. As to Deine who will attack whatever is closer to the exit. While Deine may have lower HP at 3894 she has much higher Def at 861.
That is why they are like that.