Sousuke's Breakdown Is Operational, Not Just Romantic
SPOILER!
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Recall Order · Lost Routine · Arbalest Failure · Hong Kong
Removing Sousuke from Kaname's protection detail does more than separate him from someone he cares about. MITHRIL accidentally removes the structure that had been holding his two lives together.
At Jindai, protecting Kaname gave his civilian life a military purpose he understood. At MITHRIL, returning to Kaname gave his missions a civilian destination.
What changed
Once those two halves are forcibly separated, Sousuke becomes worse at both. He cannot focus on operations, he resents the Arbalest, he loses to Clouseau and eventually walks away from the Hong Kong mission.
Why it matters
Kaname is not simply a romantic distraction from soldiering. By TSR, she has become part of the reason Sousuke knows what he is fighting to return to.