If you are looking for a beefy tank to tank physical damage, Engelbert is the man to go to. If you have got Vow of Love and set it on Agrias, she would be a little bit of a sub-tank for both Phys and Mag damage (more towards the Mag side if you have her main job also set as her sub-job), but nothing spectacular. She really is just a damage dealer that has the capabilities of being a sub-tank.
If you wanted a physical tank, Engel is the one to go to. If you wanted a magic tank, Rain would probably be the one to go to. If you wanted a tank with a little bit of both sides, then WoL or Agrias would be your go to; WoL being the better than Agrias in that aspect due to being able to have Spellblade as his sub-job in addition to his main job to be able to wield skills for both Phys and Mag,
Only one turn tho. Its not something you can depend on .
You lost one of her best skill, Royal Guards Protection as it valid for 3 magic attack, by choosing paladin as her sub job.
Its important to note that she isnt the best tank. She can buff, inflicting ailments, and taking hit more than average but overall they made her lacking a tank aspect. Its like you are comparing apple w/ banana.