Sumner Sloan is Diane's fiancé at the start of the series. He is portrayed by Michael McGuire.
Diane was Sumner's teaching assistant; she had known him for two years. Sumner and Diane stopped in at Cheers to have a drink, and to allow Sumner to retrieve his ex-wife Barbara's wedding ring, which he intended to give to Diane. Sumner and Diane were on their way to the airport; their plan was to travel to Barbados for the wedding.
Sumner left Diane at Cheers while he went to retrieve the ring. Although he promised to be gone only a few minutes, he did not return for over an hour - and when he did return, he began to have second thoughts about the wedding. Diane managed to talk him back into going to Barbados, but a phone call from Barbara and Sumner was gone again... this time for good.
In "Sumner's Return", he returned seeking Diane's forgiveness - and unknown to her, also to see if he could win her back, as his reunion with Barbara had failed. By this time, Diane was with Sam, and Sam guessed Sumner's intentions before Diane did. Failing at winning Diane back, he exited with as much grace as he could.
In his last appearance at Cheers, in "I Do and Adieu", he returned to inform Diane that an old manuscript he had of hers had been submitted to a publisher, and the publisher wanted her to finish it for potential publication. By this time, Diane had become engaged to Sam. Sumner managed to talk Diane out of marrying Sam, playing on her fears that Sam would drag her down to the point where she would be unable to finish the book. It was never revealed what happened to Sloan after this.
In addition to these episodes, he was also mentioned in Season 4's "The Groom Wore Clearasil".
Though very intelligent, Sumner could be seen as either a bit of a cad, or so detached from his own emotions that he ended up hurting Diane very much, and by association Sam.