Ishii is reliable but if anything, she's more of a filler. She's not that player that will increase the chances of a team to make it far. She wasn't even able to push Hisamitsu to the top as a captain, they sank. Her spike speed tends to be slower and then her attacks become overly predictable. Shinnabe was the one who handled all the tough attacks.
Yes, "Leader" is not a term one can apply to Yuki