Marumo no Okite

Takagi Mamoru is a bachelor around 40 who works at a stationery maker. He becomes a surrogate father to the twins, Kaoru and Yuki, when his good friend passes away. Now orphans, the level-headed, cheerful Kaoru and her younger brother brace themselves and begin a new life with Mamoru.
The family has a secret, which is the miniature Schnauzer, Mook, that the twins had picked up one day. Mook speaks human language! While he tenderly watches over the young twins and is their hero, he is at times harsh to Mamoru who makes an unreliable guardian. At other times, he encourages and even moralises. When Mook first started to talk, Mamoru suspected that it was his own mental state. He grows to accept Mook's presence as the one that brings the family together.
(Source: Crunchyroll)