Rurouni Kenshin OVA: Trust & Betrayal (Tsuioku-hen)

Taken by slavers when he was a child, Himura Kenshin is rescued only when an encounter with bandits kills off everyone but him. He is found by a master of the Divine Justice School of Swordmanship, a school so deadly that to train in it means death for either the master or student—there can only be one master. Taken by the master, Kenshin is trained in this school, only to leave before finishing so that he may join the Meiji Restoration and help prevent further tragedies like his own. Thus is born the Battousai, the greatest strength of the Ishin Patriots, a boy of 15 who kills for the sake of building a new, better world. One night, he comes across a mysterious woman, Tomoe. He must hide with her when the revolution stumbles. They marry for appearances, but soon fall in love. Tomoe has another reason to be with Kenshin, one she regrets but cannot stop. Revenge must be satisfied, and only blood can do that.

Synopsis from Anime News Network