Namesakes
Bishops of Rome and the Popes:
2 popes
Marcellus I 308-309 
Marcellus II 1555
Fictional Characters from Plays and Musicals:
1 character
Marcellus Washburn 1957 The Music Man
Notable Athletes:
1 boxing
(boxing) Muhammad Ali (a.k.a. Marcellus) 1942-2016
Saints:
5 saints
Saint Marcellus I 255-309
Saint Marcellus the Centurion ?-298
Saint Marcellus ?-304
Saint Marcellus of Apamaea ?-389
Saint Marcellus ?-510
Shakespearian Characters:
1 character
Marcellus Hamlet