A Chrono Trigger Point to Ponder – Gurus' Gambit
Norstein Bekkler is rather amazing, if you consider it. Not only is he is able to make perfect clones of the characters, but also deliver them to the particular character's home, no matter which time period they live in. That seems to combine Melchior's abilities as the Guru of Life with Gaspar's abilities as the Guru of Time.
So, consider if you will –
Gaspar, viewing the various possible time streams, realizes that the only one that isn't completely ruinous to humanity and the entire planet is one where Crono is improbably drawn into his role as a time-spanning hero by running into Marle and following her through that first inadvertent trip through time. He confers with the other gurus through mystical means to explain what he discovered. Melchior is alerted to the ultimate need to reforge the Masamune in the Present, Belthasar knows to create the Wings of Time in the Future and set his plan for Death Peak in place, and Gaspar himself completes his Chrono Trigger, and does what he can to make the End of Time somewhat more functional and hospitable.
At the proper moment, Gaspar manipulated time just enough that he could whisk Marle's Pendant off during her collision with Crono.
Melchior snuck in, under another subtle time manipulation, to align Lucca's Telepod to interact with Marle's Pendant in such a way as to 'lean' on a temporal weakness Gaspar discovered between the Present and the Middle Ages. They knew that they were tinkering with Time and Life in that Marle would likely be mistaken for Leene, and then erased from the time stream after Leene was not returned. If Crono did not figure it out, Leene, Marle, and the entire unfolding of the best possibility for the 'best' future would be irretrievably lost, but they reasoned the need was great enough to warrant the risk.
So, Norstein Bekkler could be an advanced construct of Belthasar's, who imprinted the construct's mind with limited versions of their abilities (Melchior's maker abilities to create perfect clones and Gaspar's temporal ones to deliver them in various time periods) in a far-reaching bid to help Crono and his friends save the world from Lavos.