>>528398708
"Healers" in this game aren't actual healers. They are shield-regenerators. They can only "heal" DP, but not HP.
Tama's only really unique that she can "revive" DP. But you can probably imagine why Aoi would've still died even with the presence of Tama.
Her death was due to seraphim overuse. Tama can only revive shields.