Dr. Bruce Sumner is a psychiatrist and the Director of St. Claire's Hospital, where Walter Bishop was committed for 17 years. He is opposed to Walter's release, claiming that Walter is mentally unstable, and a danger to himself and others. In subsequent encounters with Walter, Sumner appears to be highly motivated to get Walter back into St. Claire's; it is not clear whether this is for personal reasons, or strictly professional.
Sumner appears to have no formal connection to Walter after Walter is released under Peter Bishop's legal guardianship.
Olivia Dunham attempts to interview a patient at St. Claire's and is blocked by Sumner, who negotiates a compromise where he will only allow Walter to conduct the interview. Peter is opposed but Walter volunteers to go back. During the inteview, Sumner uses Walter's agitation as cause to sedate him and hold him for evaluation. It takes an overnight stay and a court order to get Walter released. Afterwards, Sumner mentions to Peter details from Peter's shady past and says he will use that to petition the state to revoke Peter's guardianship of Walter. ("The Equation")
Sumner appears to have a supervisory role over Walter's release, which includes regular evaluations and recommendations.
Sumner conducts an on-site psychiatric evaluation of Walter at the Harvard Laboratory, pressing Walter about his apparent recurring hallucinations. When Walter lies by saying the hallucinations had stopped, a skeptical Sumner urges him to be truthful, so "I can make recommendations for your continued care, whatever that may be." Walter sticks to his denial. Phillip Broyles later shows up at the lab to check on Walter, telling him that Sumner had called him with concerns about Walter's condition. ("Alone in the World")
Walter accidentally discovers a psychiatric evaluation form sent by Sumner to Olivia (in the Alternate Timeline, Olivia was responsible for Walter's release), requiring her to recommend whether he should be re-hospitalized. The form indicates that, in Sumner's opinion, Walter should be readmitted for "long-term evaluation." Olivia recommends against it. ("Subject 9")