Mifune seems to be a typical by-the-book soldier. Before the Battle of Zion begins, he is shown coordinating the defences on the dock and he is initially hesitant to allow the Kid to assist the APU corps as an ammo reloader since the Kid is under 18. Kid persuades the captain to allow him to volunteer by saying that the Machines don't care about his age, that they'll kill him all the same. Minutes before the Machines attack, Mifune gives a rowsing speech to motivate his troops.
"All right! This is it! Now you all know me, so I'm gonna say this as simply as I can. If it's our time to die, it's our time. All I ask is, if we have to give these bastards our lives... WE GIVE 'EM HELL BEFORE WE DO!"
Captain Mifune makes a valiant last stand against the Machines as the dock is overwhelmed. As he calls for a reload, a massive swarm of Sentinels gather and prepare to advance on him. The Kid struggles to reload the captain's APU guns as Sentinels close in, but finally manages to insert the ammo cache. With or without the extra ammo, however, there are simply too many Sentinels for Mifune to defeat on his own and the robots swarm all over him, slashing his face multiple times with bladed appendages and flaying him to death.
Before he dies, Mifune tells the Kid to take his APU mech and open the third dock gate as the hovercraft Mjolnir is on its way back. Kid says he never finished the APU training program, but Mifune manages to utter "Neither did I" before succumbing to his wounds.