I pulled on the Mythic for Guard Bearing fodder and got a random Triandra.
I don’t know anything about AR but I started taking it more seriously lately.
I guess my Lute would aappreciate the fodder, and I don’t like using 5*limited characters too much but I guess mythic may be an exception.
She dies to a sneeze but she’s deceptively fast and strong. Her niche lies with her dance. It’s a spectrum ruse that grants a unit another move, so unlike a typical unit with a ruse, you’re technically not losing an action to activate it. She also pairs extremely well with Yune and Bramimond who both excel when the enemy has penalties on them. But she has very low bulk and that strength means she’s easy to bait. So if you hang onto her, keep her at the back of the field.
Two questions to consider. (1) Are you interested in changing your Dark defense team? (2) How many Dark mythics you got on the team?
If you like the defense team and it’s working well, there’s no reason to hold on to Triandra if you’re not planning to incorporate her somehow. Similarly, if you have two Dark mythic units already and don’t plan to change them out, adding a third won’t change the amount of lift loss reduction because it caps at two mythics. If you are up for altering the Dark team, or don’t have two Dark mythics, then I’d say hold on to her. Otherwise, if everything’s running great and you’re at your max lift loss reduction, I think it’s safe to fodder her.
If forgot to mention it but the only mythic I own are Duma and Peony so she is my first Dark mythic
Then if you care at all about AR, keep Triandra. The lift loss reduction is incredibly significant and she’s a really good option for defense teams.
If she’s your only or second Dark Mythic, she’s worth keeping for Lift Loss Reduction if you want to use it. It’s quite helpful if you don’t want to commit the resources to building a strong defense team 
Her dance cripples many tanks that use healing specials in order to sustain themselves. Her high Atk is a problem for if their tank is able to bait her out and attack instead of dancing, but she’s still a very capable Mythic. 
Strong but flawed.
That’s pretty much it really.
