Marcus Mouse is a poor farmer. He is the husband of Margie Mouse. Together, they are the parents of Minnie Mouse. He is described by his daughter's fiance Mickey Mouse as "stern". He seems to be a somewhat tired man, as can be inferred from his frequently being seen with his eyes only half-open.
He also seems to be an easily swayed individual. For example, he first rejected Mr Slicker's proposal that he influence Minnie to marry him in return for money, but changed his mind not long after when he realized just how in need of the money he was. Another incident that demonstrated his pliability was when he accused Mickey of stealing his eggs shortly after calling him a "great little feller" for offering to help him financially. Then again, it is worth noting that both of these incidents occurred when Marcus was in deep financial trouble and was close to losing his home to Squire Skinflint, so it might be true that he would not change his mind in more normal situations and only did so out of desperation.
Behind the scenes
Marcus Mouse first appeared in a strip of Mr Slicker and the Egg Robbers that was originally released on October the 25th, 1930. He played a major role in the story, one of the few times he would ever do so. Just a little over a month later, he appeared in A 'Pot' Shot as Minnie's "stern papa". Interestingly, he was depicted without a beard in A 'Pot' Shot, which stands in contrast to his Mr Slicker and the Egg Robbers incarnation with a long, white beard.