Flaky, perhaps - Of a person: liable to act in an odd or eccentric manner (as though exhausted or under the influence of drink or drugs); crazy, ‘screwball’; feeble-minded, stupid (OED).
Or,
Unpredictable - difficult or impossible to predict (accurately) (OED).
Or,
Capricious - guided by whim or fancy rather than by judgement or settled purpose; whimsical, humoursome (OED).
Or,
Unsettled - not settled or staid in character; of a restless or turbulent disposition (OED).
Or,
Changeable - that may change; liable or subject to change; mutable, variable, inconstant (OED).
Or,
Skittish - fickle, inconstant, changeable; tricky, difficult to deal with or manage (OED).