Corporal


 * Our Hero/B.J. Blazkowicz (Corporal in Doom II strategy guide. Generally assumed to be Corporal based on character from the novels, and that he was ordered to remain behind, he is promoted to Sarge if all secrets are found in Doom RPG, and promoted to commander as per Final Doom.)
 * Marine (Doom II)
 * Marine (Doom III)
 * Flynn Taggart
 * Arlene Sanders (Lance Corporal, after she is promoted later in the books)
 * William Gates
 * Corporal Stout
 * Corporal Ryan
 * Albert Gallatin
 * Adolph Hitler ("Austrian corporal")
 * John Grimm
 * Dean Portman

Behind the scenes
The hero of Doom 1-3, and RPG is a Corporal based various sources. Assuming no 'promotions or demotions' (beyond best score in Doom RPG). The main hero of at least one of the campaigns in Final Doom is Marine Commander implying a higher rank than Sargeant (Sarge).