Summary
Senior Petty Officer Frederic-104 was the commanding Spartan during the Reach campaign. He was one of the few Spartans that survived both the Fall of Reach and the First Strike missions. He was the second highest ranking active Spartan to the Chief and the two shared many similarities in personality.
Frederic-104 appeared in novels Halo: The Fall of Reach and Halo: First Strike. He is one of the SPARTAN-IIs, in the same batch as the Master Chief, John-117. He took part in many missions alongside the Chief and was leader of Red Team, which was ordered to defend the groundside reactors from Covenant attack. He linked up with other Spartans and was retrieved when the Chief led a group of UNSC survivors that included Vice Admiral Whitcomb and Sergeant Johnson to look for Spartan survivors. He took part in and survived the First Strike mission to blowing up the Covenant command-and-control centre Unyielding Hierophant.
Where he is during the events of Halo 2 is unknown.
(possibly fighting covenants on Earth.)
Speculation: Perhaps a novelisation of Halo 2, if it is ever released, may provide the answer to this enigma.