Elixir of Immortality.
In essence, it is an advanced bloodline enhancement material. Once consumed, the user's form of life will undergo extreme sublimation, finally locking into the "Celestial Maiden" template.
Since it's specifically for a Celestial Maiden, Gu Mo, who has no dissatisfaction with his gender, naturally would not consider using it on himself. Moreover, as there is only one portion of this elixir, no matter what, it is naturally best for Kikyo to use it. Her improvement in strength also benefits him.
After all, the witch is currently the only fixed teammate...
Hiri, of course, is not considered a teammate. Firstly, because she still can't stabilize, it's only her first time opening the gate and following along. Secondly, because the Demon Hunter Girl's combat strength is not impressive enough...
