What are more characteristics of Otter?

Otter is a lower-class "Cockney" type character. He is sociable, resilient, and plugged into community doings. He is smart, and the other characters appreciate him because he can always be relied upon to be helpful. As Mole puts it, everyone likes him.
Unlike the main characters, Otter has a family, including a young son named Portly. In chapter 7, we learn from Rat that Portly is missing. Through Rat's narrative we learn more about Otter. For example, Otter is not the type to get nervous or to let his feelings and fears show. Since Otter is now openly nervous, Rat knows that Portly's disappearance is serious.
We also learn how much Otter loves his young son when Rat and Mole find and return him to his father. Otter has been waiting, "tense and rigid," but when he sees his son, he makes an "amazed and joyous bark" and bounds forward happily.