John 117, The Father of the Clones

After destroying the Halo ring and the Flood, Master Chief ends up in the Star Wars universe due to his half of the Forward Unto Dawn being pulled into slip space. There he is found by the Mandalorians during their civil war. After helping the side that saved him and Cortana win, he becomes a mercenary and is eventually recruited to be cloned on Kamino instead of Jango Fett.