Complex sentence: The 6 friends suspect that Krei is the masked man but it is revealed to be Callaghan, who explains that he started the fire that killed Tadashi to steal Hiro's microbots.
Split into simpler sentences: The 6 friends suspect that Krei is the masked man but it is revealed to be Callaghan. Callaghan explains that he started the fire that killed Tadashi to steal Hiro's microbots.