The wisecracking loudmouth of the Reds, Grif is self-centered, acerbic, and lazy. Despised by Sarge (and basically everyone), he has the lowest social and military rank on the Red Team, though did possess the rank of Sergeant just like Sarge at one point during Reconstruction. He and Simmons share a love-hate relationship, and often bicker or chat with each other about the various situations that the Reds encounter. Although Grif's laziness does create problems for the Reds, Sarge always blames him when things go wrong, regardless of Grif's (or any other team members) actual culpability.
With respect to personality, Grif is fairly brash and has a flair for melodrama. He is quick to make silly theories or insults, even toward people who could clearly kill him without hesitation. He is also a coward, panicking and fleeing at the first sign of danger. Additionally, he has almost never used his gun (one rare time being in Season 2) and, quite possibly deliberately, mispronounces words like "both" ("bolth"), "margin" ("margarine"), and "envelope" ("involope"). Grif is, however, definitely not stupid, showing far better common sense than many characters throughout the series, commonly displayed by stating the flaws in Sarge's plans.
Although Sarge and Simmons regard Grif as unintelligent, the likelihood is that Grif simply has a bad habit of speaking or acting before thinking. In fact Grif is capable of being more responsible than he shows; when Simmons defected and began to wage a one-man war against his former teammates, Grif was the only member of Red Team who tried to actually solve the problem. Despite his constant flippant disregard for Sarge's authority, Grif seems to care at least a little about Sarge, as he was quite upset when Caboose almost killed Sarge.
Physically, a lifetime of cigarette smoking and over-eating has left Grif badly out of shape for a soldier in the military, and he is barely able to run 300 feet without running out of breath. He claims he's eaten nothing but Oreos for years, and nothing to drink but ketchup and gravy for months. After being accidentally run over by Sheila, driven by Tucker, Grif had to have most of his body parts replaced with donor organs left over from Simmons' cyborg operation. Even this event, however, has not compelled him to quit his bad habits. His less than athletic physique and capability is something of a joke, as Simmons once commented during Revelation "Grif, you're so fat, one time you broke your arm and gravy came out."
Grif, unlike Sarge, is not a very capable fighter. He has displayed during his fight with Tex in chapter 10 of Revelation that he's not as strong as Tex, as shown where he managed not to hit Tex even once at close range with an assault rifle.
During season 9, where most of the characters had edits done to their personalities and habits due to the corruption of Epsilon's memory, Grif is radically different as well. At first, he was shown to be a clean-freak. He was obsessed with working hard and keeping everything nice and tidy and, upon being told to take a break, he hadn't at first recognized the term and didn't understand how to take a break. However, shortly afterward, Grif began taking breaks frequently and is now seemingly back to normal. He also showed a habit of doing things three times, like counting to one hundred, turning on light switches, unlocking doors and even using phrases; whether this is a result of Epsilon's memory being corrupted or if he truly does possess this habit is currently unknown, though it is likely that he may only possess it in the Epsilon unit due to the corrupted memory.