Zaa~ko is a meme which origins from Yuzu-Soft's game Tenshi Souzou Re-boot!. One of the main heroines, Tanikaze Amane teases her brother, a former demon lord, who lost his power by calling him a zako. It can be translated as a "small fry" or "prey", something really small and weak, like level 1 monsters. "Loser" can be a close english equivalent.
Amane is a mesugaki who loves her brother dearly, when teasing him she's blushing, smiling widely, showing her smug face, presenting a fang and looking directly in the eyes. Amane often seeks a physical contact with hearts flying around her.
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