sudden, abrupt, improvised
俄 centers on something happening without warning: sudden, abrupt, or improvised on the spot.
俄 combines 亻 (person) with 我, which likely contributes the sound and may relate to the idea of a sudden, self-assertive action. The exact historical development is uncertain.
A person 亻 suddenly acts on their own 我, doing something abrupt and improvised without warning.
For ガ, imagine a person suddenly shouting 'GA!' in surprise: the sharp sound cues ガ and matches the abruptness of the kanji.
sudden; abrupt; unexpected; immediate; hasty; improvised; impromptu; offhand
suddenly; all of a sudden; abruptly
suddenly; abruptly; unexpectedly
bandwagon fan; fair-weather fan
mushroom millionaire
impromptu skit; (theatrical) farce; extravaganza
sudden blindness