Fujinmon is the first-built son of King Drasil & QueenChessmon, & also the older brother of Kapurimon. He fell in love with Crusadermon, daughter of Drasil's arch nemesis. Knowing their marriage would never be approved of, the two of them eloped.
When news of Drasil's death reaches him, Fujinmon returns to take care of his mother, depose of the interloper Bombmon & take his rightful place as the Machine King.
Fujinmon notably brainwashes Alexander Musha into temporarily thinking that he is the King of the Machine Empire & that the other DigiDestined are his enemies. Another time, he traps the people of Earth in a time loop, forcing them to relive the same day over & over so that he can observe the best way to attack them. Unfortunately for Fujinmon, Alex knows there is a time loop thanks to Fujinmon's earlier tampering with his brain & is able to use that knowledge to put a stop to the plan.
When Drasil is rebuilt & returns, Fujinmon & Crusadermon flee temporarily, but then they come back in the episode "Hawaii Zeo", where they grow into giants but are overwhelmed by "Zeomon + Pyramidramon + Agunimon". But the two survive the blast (though they shrink back to normal size), & just as they are about to face Kapurimon, King Drasil arrive to confront them. With that, Fujinmon & Crusadermon flee & are never seen again.