Complete all maps at 100% and you will unlock stage 36 (Ichigo vs. Ulquiorra). Beat this stage and link Ulquiorra with Ichigo. Now you can press L to transform into Hollow Ichigo.

First, complete all the maps at 100% to unlock stage 36 (Ichigo vs. Ulquiorra). Beat Stage 36 and Equip Ichigo as a supporter. Link with Ulquiorra and press L to change form into Ulquiorra.