Well, I would say it depends... If you are going to use a Miracle Ticket I would grab Osiris (Heal+15 burst for ally) now and wait for Pluto.
Then you could run Jeanne with Ambush and you would reach -40 attack and -40 def down, well with Thanatos you would reach -50% now.
But later we will get Satan Awakened and she offers -10% dark resistance down (really awesome because it's decreases debuff resistance) and -15 def down (A-frame like Ambush). So basically -25 def down. I would grab her later if you don't have her until the next Miracle Ticket.
But if you don't want to spend your money on a miracle ticket then try your luck with Thanatos now. She is also really good with her C-frame (-20 def down) and Satan's awakening will come out in August or September.