Born in 2064, Walter's family were some of Huffman Island's first immigrants, with Walter growing up in Freedom City along with his two best friends Randy O'Niell and Glen Duval. When the 1st Huffman Conflict broke out between the USN and OCU, the three were caught up in an attack on the city, leaving Walter with a laceration scar on his left cheek. After the war, Walter and Randy were forcefully repatriated by the USN while Glen was repatriated by the OCU. While at the USN orphanage Home 47, Walter met Lynn Wenright, a quiet girl who quickly fell in love with him, but he never took much notice of her and forgot about her soon after leaving the home and joining the USN army along with Randy.
Many years later, Walter and Randy would reunite with Glen as apart of the USN peacekeeping force sent to Huffman during the Huffman Crisis in 2086. Upon seeing Glen piloting a wanzer, Walter and Randy signed up for the USN Army's wanzer pilot training program. As tensions between the OCU and USN rose following the Larcus Incident, Walter and Randy qualified as wanzer pilots, and were sent off to fight in the 2nd Huffman Conflict soon afterwards.
After the 2nd Huffman Conflict ended, Walter applied to be reassigned as a member of the Strike Wyverns, serving under Lynn on the USN aircraft carrier Eclipse. While there, Walter met Edward Collins, who would replace Randy as his squads mechanic. During the briefing, the team hears about the destruction of a missile facility as the group splits into two: Airborne Raid and Assault Landing. If the player chooses to join the Airborne Raid, the team is airdropped by the transport aircraft, otherwise the team is deployed offshore via landing crafts. Near the end of the missile facility assault, the teams return to the LZ to evac via transport helicopters. Unfortunately, all of the transport helicopters are shot down by the Garsade's cannons.
Two months later, he is reassigned to the Barghest by Hector Reynolds at the Fort Monus Base. He is sent to Manaus Airport, where Hector explains to him that a a passenger jet has taken over by terrorists, who are holding the passengers hostage. The unit arrives too late to stop the jet from taking off, and they learn it crashed over a forest in Peru, South America. They travel to the crash site and Walter sees the cargo hold is still intact, and comes face to face with Morgan Bernard. Walter later serves in a mission to Cambodia, where he sneaks on a cargo train containing the T-1 Brutal Wolf and is attack by Grimnir Wanzers. Walter manages to escape, and witnesses the Brutal Wolf being carried off by helicopter.
Returning to Fort Monus, Hector tells Walter about MIDAS and the Barghest is sent to Alaska, believing the Reactor Complex is the Grimnir's next target. Walter and the Barghest are forced to fend off waves of Grimnir wanzer until Lynn arrives with reinforcements and a new prototype wanzer, the Gracilis. Walter then confronts Morgan again and the two battle, while Emir has MIDAS carried off by a transport plane (as seen in Front Mission 3). Despite managing to defeat Morgan, the remaining Grimnir forces, led by Glen, infiltrate the Complex and attempt to force the reactor to overload. Walter is forced to battle his childhood friend and, after a long and strenuous fight, emerges victorious. After being hit by Walter's EMP system, Glen is temporarily cut off from the S-Type Device and returned to his senses. Accepting his fate, Glen says goodbye to Walter before being vaporized by MIDAS, with Walter being saved by Lynn in the nick of time.
In the epilogue, Walter is promoted to Colonel and retires in the USN. While visiting Lynn's grave, Walter is approached by his daughter, who tells him that she qualified to be a wanzer pilot just like he parents.