Grace adopts him during the first season despite Will's objections. Will quickly become attached to the dog, but they both find that they cannot devote enough time to look after him. They ultimately give the dog to Jack. 
He is last seen in the third season when Jack briefly moves back into Will's apartment (bringing Klaus von Puppy and Guapo with him) before eventually moving to apartment 9A across the hall. Neither Klaus von Puppy nor Guapo are seen or mentioned after this episode.