For your information, Man-Machine Interface Level 4 isn't a widespread feature amongst the M15 Kishi because when Nimura devised it, it amounted to a self-destruct mode.
MMI-4 has a Kishi uprate its wearer to handle the computing of a quantum computer. For most users, that's impossible with what they have, which means that the Kishi has to upgrade you. The upgrade process eventually turns out fusing you with the power armor, preventing you from continuing to function like a human being. In return, you grant the power armor the features it lacks: gyrospopic control posture, balance, grace, skill... the kind of things that allows it to trancend just being an armor.
Reika is a NH-33 Eihei. She's one of the few rare cases where Shinigami recalled the stuff that was experimented and discarded through the prototype stages, and decided to use it anyways because it figured that it could briefly flip the switch on and off to gain the kind of man-machine communion Reika needed to win the fight. And, of course, since that made Reika addle-brained through the process ("this hurts, what is happening to me?") Shinigami "assumed direct control".
At least until a point before it crossed the line of no return, before it rewrote Reika so completely that she stopped being Reika. Nor that her body be changed so far that it wouldn't have a chance to revert back.
Understandably, though, Reika currently feels like crap.