Yami true form..

[Kaizokon]

Well till now,the number of espada are decreasing..
Bsides of the top 3 espada, only Ulquiorra and Yami are alive..
What is Yami probably doing now and where he is?

[(SIC)NESS]

yami is not in the top 3 he is espada 10 so he'll get his ass kicked anytime soon and ulquiorra is number 4

[Houzukimaru]

reading comprehension much? he said BESIDES the top 3 only Ulq and Yami remain

as for the OP this question has been asked many many times before and no one really knows, a couple theories floating around are that his soul sucking ability has something to do with Aizen's plan to make the Ouken so hes being kept on the side, or that when Aizen told the Espada to return to their chambers when Ichigo and co. arrived he did so and has been forgotten about since, not being strong enough for Aizen to bother taking with him with the top 3 or is being saved to fight someone like Chad