Kind of a longer thought about historicism in general:
As pointed out in the comments, "transitional" figures are typically chosen by historians after the fact.
There are however periods of music history where figures actually try hard to get away from the past. A good example would be the changes that happened from Medieval polyphony to what we now call the the styles of the Renaissance. The early Renaissance theorist Tinctoris even said that before the year 1430, there was hardly anything composed that was worth listening to!
There are also many figures that are so radically different for their time yet seemed to not have paved the way for development in their direction. An example of this is Carlo Gesualdo, whose harmony could be considered radically expressionistic for his time period. A great counterexample who did end up having an impact on harmony and history would be Monteverdi, who called himself a practitioner of a newer style for his day, the secunda practica as opposed to the prima practica of older composers like Palestrina. And of course there are composers who both span multiple periods, and are even responsible for their creation: Stravinsky being one of the most shining examples of this, early on being comparable to other early modernist composers (eg Bartok, Debussy), to his neo-classical phase, and finally his serial music phase.