One vote for Mifune here.
Musashi should be the fluff-tastic super unit. He was the greatest warrior in the chapter, before forsaking all rank and title to serve in a more humble role. However, when the chapter has need of him, he will leave his simple life and return to arms. Although he may take command for a time, he will return to his farm and hut on a feral world when it is all over.
Mifune was made a ronin, a disgraced battle brother, very early on in his career. However, after years of valor and heroic victories, he was accepted back into the ranks and hearts of his brothers. His rise from there was meteoric, and he is the first former ronin to ever rise in rank, let alone be chapter master. His attitude is slightly more pragmatic than his brothers, and his tactics can seem at times either weak or crude, but he always has a plan, and he always gets his results.
Best I can come up with right now.