Captian Wallace
From Halopedia, the Halo wiki
Summary
Captain of the UNSC Commonwealth. Wallace lost his left arm below the elbow some time in his career. Rather than utilize an artificial limb, he simply did not have a left arm, something that probably took getting used to by his crew.
Wallace was rather by-the-book and did not enjoy having civilians on his bridge, as evidenced by his displeasure of Dr. Catherine Halsey.
In 2525, Wallace was instrumental in helping the Spartans cripple a Covenant frigate sufficiently enough for them to destroy it.