There's not enough info around to really gauge her usefulness.
For now, she seems to give out 'buffs' and heals in form of 'drinks', which take the form of various cups, flasks and plates of which they are served on.
You can only have one buff going, though the said buff can be stacked for increased effect. But there's a higher side-effect and a chance the buff will backfire.
Also, since she is a sake bottle, there's a good chance that some of the potions are actual alcoholic beverages and can negatively effect the user when used too much.
So she seems she needs to be used 'in moderation', not to get too drunk on those effects.
Aside that, without any offensive or control abilities, Sosogu feels as a too passive vessel to have around when fighting, thus questioning herself.