A being entirely overtaken by Elemental Water, whatever personality Nishi had before is effectively void. Her blithe and carefree nature pushed to its limits to the extent she can spend entire days completely oblivious to reality around her.
This empty-headedness shouldn't be taken for safety, however. Nishi is an incredibly skilled combatant even without her water-enhanced form, and it is still possible to get on her bad side, or the bad side of whatever's currently occupying her.
Her birthday is January 16th
Her favorite food is Watermelon
There's still a sensible fragment inside her somewhere ,but she tries to keep it down.