[1] He was probably a friend of Dante Alighieri who made him into the main character of the 2nd canto of the Purgatorio (the second part of the Divine Comedy).
All that is positively known about him is what is found in Dante's work and it has been impossible to identify him with absolute certainty with any of the Casellas named in contemporary documents.
To whatever is said of him in Dante's work one can add (with some degree of probability) information furnished by the earliest commentators of the Divine Comedy: Pietro di Dante, Benvenuto da Imola, Buti and Landino give him as being born in Florence, while an anonymous early commentary of the Divine Comedy gives him as being born in Pistoia.
Specifically, in line 107 of the Canto II, it might be inferred that the amoroso canto ("amorous song") that Dante connects with Casella is a specific indication that Casella's music was (at least in part) in the monodic style which accompanied Occitan lyric poems, or Italian lyric poems in the Occitan manner.
Casella begins to sing the first lines of a Amor che ne la mente mi ragiona, a poem written by Dante himself.