I'm Not Sure If the Cat I Found Is Actually a Cat.

On her way home after being dumped by her boyfriend of seven years, Kaoru Honda picks up a black lump that appears to be a cat. However, its true identity turns out to be a strange creature that speaks human language, enjoys drinking alcohol, and insists he is a cat. Kaoru names him "Modoki" and decides they will live together. As they share a home, her dreary daily life gradually begins to change.

(Source: AlphaPolis, translated)