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- “Nothing can kill the Chief.”— A young John-117 refers to Mendez
He was the Spartan-II's training leader during their first years on Reach in 2517 until 2525. He trained them in the aspects of combat as hard as any Marine when they were only at the age of 6. His military training was punctuated with in-class strategy sessions with Déjà.
In the beginning of their training, Mendez and his training instructors used Electric Batons to discipline trainees. However, his trainers would occasionally use rifles with tranquilizer rounds to incapacitate the trainees (those that would eventually make up Gray Team) who attempted to escape. Eventually, the Spartans, Spartan-117 in particular, grew to look up to Mendez as a solid, unmovable force in their life.. They had great respect for him, perhaps even to the extent of seeing him as a father figure.
After the SPARTAN-IIs graduated, Mendez was slated to train the next class of Spartan-IIs, but the program was delayed indefinitely by Dr. Halsey due to issues with viable candidates. Mendez requested reassignment to active duty. Over the next six years, he fought the Covenant on five worlds, won two more Purple Hearts, and was promoted to Senior Chief Petty Officer. Mendez was then recruited by Colonel James Ackerson to assist Kurt-051 in running the SPARTAN-III Program. He trained three SPARTAN-III companies at Camp Currahee on Onyx from 2531 until 2552. He was originally skeptical of the job, comparing the behavior of the trainees to that of the schoolchildren in Lord of The Flies.
Battle of Onyx
- Main Article: Battle of Onyx
Mendez was on the Shield World Onyx when it activated. He, along with SPARTAN-III Team Saber, Tom-B292, Lucy-B091, and Kurt-051 were virtually alone on the planet, and were forced to hide from Sentinels until Kelly-087 and Dr. Halsey eventually arrived, followed shortly by Fred-104, Will-043, and Linda-058. Mendez assisted them in fighting all the way to the core of Onyx. Mendez then evacuated to the Micro Dyson Sphere in Onyx's core along with the other survivors. At the funeral for the Spartans who died in combat, he read an excerpt from his favorite book: an account of the Interplanetary War called A Soldier's Tale: Rainforest Wars .
Mendez is a strict, by-the-book military man and thus has a solid sense of duty and honor. He also rarely allows himself to display his emotions (for example, John-117 very rarely had seen him smile). When he heard of the destruction of the Spartan-IIIs, he kept his face stony, holding his feelings to himself. His total need for discipline and order off-sets many of his new acquaintances, however those who know him well (namely the Spartan-IIs, particularly Master Chief and Kurt Ambrose) have a respect and relationship with him that can be compared to that of the connection between father and child.
Mendez is a battle-hardened veteran who has won numerous decorations, including multiple Gold Stars, Silver Stars, Bronze Stars, three Purple Hearts, and The Red Legion of Honor.
- Halo: First Strike has a potent reference to Mendez, in which Dr. Halsey finds a string: CPOMZ, followed by a 512-character alphanumeric string. It is later confirmed in Halo: Ghosts of Onyx to be a reference to Mendez: Chief Petty Officer MendeZ (CPOMZ).
- He had his own custom-made Warthog which is equipped with a rack of Argent V missiles instead of the standard M41 Vulcan Light Anti-Aircraft Gun.
- He reads the book Lord of The Flies sometime in his life and compared the Alpha Company recruits to the kids in the book in a private conversation to Kurt-051.
- He enjoys Sweet William Cigars.
- Mendez gave John-117 his quarter back at the final moment they met. He recalled that John broke a few of the trainers' fingers when they tried to take the coin from him.
- In Halo: Ghosts of Onyx, Kurt-051 mentioned that Mendez aged very well, that he had a trim chiseled body from "ironwood", with hair that was now silver, deep creases around his eyes and that a set of ragged scars that ran from brow to chin.
- "A leader must be ready to send soldiers under his command to their deaths."
- "You don't win unless your team wins."
- "Do they let Spartans drink now?" 
- "Start tumbling in this gear, start praying."
- Halo: Ghosts of Onyx page 69
- Halo: Blood Line
- Halo: The Fall of Reach, Chapter 4
- Halo: The Fall of Reach, page 38
- Halo: The Fall of Reach, page 35
- Halo: The Fall of Reach, page 99 - "The only constant in the universe"
- Halo: The Fall of Reach, page 99
- Halo: The Fall of Reach, page 101
- Halo: Ghosts of Onyx page 366
- Halo: The Fall of Reach, page 95
- Halo: Ghosts of Onyx, page 70
- Halo: The Fall of Reach, page 69
- Halo: The Fall of Reach, page 62
- Halo: The Fall of Reach, page 42