This character got released today along with Goltarion. The both have absolutely broken tech. At full cap, Goltarion is a soldier chick. He doesn't stay dead, period. He will always revive and down for some more tanking.
At full cap with Lv. 13 buff/debuff, Eternity is the tankiest DR tank in the game. With 93% innate DR, 95% status resist, infinitely stacking DEF buff, and 50% HP revive in devotion mode. Anything that doesn't ignore DR does nothing to her. Trying to apply status effect is almost impossible. The stacking DEF buff is almost like an afterthought. Her 50% HP revive means she doesn't require fixing if she go down. Additionally she gives the one she is protecting 104% ATK buff, the best ATK buff in the game, 65% DR, and 2 damage nullify.
When angry, she removes 101% of enemy DEF on A1, deals essentially double damage on A2. Almost doubled ATK, 97% DEF PEN, and counter with 150% of ATK, the best counter in the game.
Eternity is nuts. Her only drawbacks are she only protect one unit and can't have the best of both anger and devotion. Also Heavy SS Defender is a resource nightmare. Andvari will hate you if you try to craft her.