Host of the titular Youtube channel, in which he does Let's Play-style commentary over various games, infamously horror titles. Guest-stars in Betrayal at the House on the Hill and Shadows Over Camelot.
Chronic Backstabbing Disorder: Contemplates abandoning the survivors and join Arin and the cannibals. When it looks like the traitor is going to lose, he acts as if he never even considered the idea. He's also The Traitor in Shadows Over Camelot.
Despite the fact that he is an animator who makes a living as a professional freelance artist, he is utterly terrible at pictionary, and all of his drawings are completely off-the-wall incomprehensible.
No Pronunciation Guide: When asked to draw cologne, he assumes it's pronounced Col-log-ne, and has absolutely no idea what that means.
Professional Killer: He describes his job as "to kill people, then find the time on his watch". Apparently he's killed 10 people.
The Klutz: Almost as soon as the episode begins, he drops and breaks the Pocket Watch prop he was given to use during the show. He then puts the pieces beside each other andclaims it's fixed.
Hair-Trigger Temper: He has a generally sarcastic and pissed-off tone through the entire episode.
Large Ham: After Barry and him win a round by guessing "Embrace" he freaks out and runs to shout at the camera man.
Not Even Bothering with the Accent: Unlike all the other guests so far, Stamper doesn't even try to sound like a gentlemen from the past. His utter lack of caring about any historical theme or consistency leads to everybody else Corpsing and abandoning their historical accents as well.
The Faceless: He always walks onscreen in front of the table, and when he reaches the table his back is always facing the camera. When he addresses the camera directly in One Night Ultimate Werewolf, his face is concealed in shadow.
The Stoic: The only times he every speaks are to answer the question they call him in for (and he uses extremely short sentences).
Unusually Uninteresting Sight: Despite being a menacingly cloaked, unnamed character who shows up out of nowhere with no context given for his job or presence, most people seem to treat him with casual indifference. Danny is the only person so far who has been freaked out by the oddity of it.
Other Real People (And Animals)
Brian Wecht (Ninja Brian)
The other half of Danny's band, Ninja Sex Party, as the silent Ninja Keyboardist. In reality, he's also a theoretical physicist. He's been mentioned on the show many times, and has appeared in Ross' advertisement for Steam Train, but has yet to speak on the show. Danny has hinted he may be featured on the show at one point. Has also composed the music for the Mycaruba Shirts promo video.
Alter Ego Acting: Zigg-zagged. Originally they would talk about him more in-character, for example joking about him being psychotic and being able to teach them Ninjutsu. Lately, this has been subverted more often then not.
The Loonie: Apparently, when asked to come up with a band name, he can up with a long list of 70. All of which were completely unusable. Examples include Placentimeter, Whitney Houston We Have a Problem, Wet, and Nope.
Magic Versus Science: Danny mentions that any supernatural claim, like psychics, piss him off because he is firmly on the science side, which is ironic considering in-character Brian uses psychic powers to blow up buildings. He's also pissed off by pseudoscience, like antioxidants and homeopathic remedies that claim to hold health benefits despite no evidence to support the claims. He does, however, like free radicals. Because they're radical.
Masochist's Meal: He guest hosts an episode of Hot Pepper Gaming, wherein he then eats a pepper whole, stem and all, silently reviews an entire game, doesn't blink his eyes for the whole duration of the three minutes or otherwise show any sign of being disturbed by the pepper he ate. Just to spite it, he then gives the pepper a 0/5 for hotness. And it was confirmed at the Magfest 12 Ninja Sex Party panel that Brian did in fact eat the pepper (unlike what a few theorized he did, believing he simply stuffed it in his mask) and the entire shot of him not blinking was NOT edited in any way.
The Omnipotent: When a fan asked how many people Ninja Brian has murdered, he claimed to be able to see the entire universe from infinite past to infinite future.
Brian: (So) the answer is infinity.
Troll: In real life, Brian is apparently this to Danny big time. Examples include 'The Beej' and the 'sexy widdle baby' story.
The Voiceless: As his character in Ninja Sex Party, they joke that he never speaks, and conveys every thought he wants to with his eyes alone.
Suddenly Voiced: During the Ninja Sex Party and Steam Train panel at Magfest 2014, he did break character and talk to answer some questions, though he did answer a few as if he were still in character.
Holly Conrad (Commander Holly)
Ross' wife and costume/ special effects artist and Maker Studios/Polaris Community Manager, with several apperances on several reality shows such as Heroes of Cosplay. Has yet to appear on the show in any capacity (excluding a brief voiceover in the Outtake reel for the Dan/Steam Train T-shirt commercial), but is mentioned often. She has stated that she enjoys the Game Grumps fans, and has expressed an interest to be on the show some time in the future.
Accidental Pun: According to Ross, she hates puns, so once when he cracked one Holly smacked his leg. When asked why, she said it was to "Pun-ish" him. This cracked up Ross' father (whom they were visiting in Australia and loves puns) and Holly was congratulated, only for her to admit later that the pun was completely accidental.
Chekhov's Gunman: Before being on the show as its first ever guest, Grant was mentioned by Jon in several previous Game Grumps episodes, including in a Banjo-Kazooie episode (where Arin brings up that Jon got an email from him) and Donkey Kong Country 2 (where Grant apparently sent them a package of fanmail originally meant for him).
Cloud Cuckoolander: Demonstrates his weirdness by sending the Grumps some fanmail he himself received and asking them to return it, much to their confusion.
Shaggy Dog Story: Tells an overly long story about his swollen testicles (although it did eventually have a point).
Split Personality: Apparently with Donkey Kong. "I am DK. I look in the mirror I see DK".
Vitriolic Best Buds: Becomes this with Jon. You can seem them chatting on twitter from time to time.
Mochi and Mimi
Arin and Suzy's two pet cats, who are brother and sister through their mutual father. They have appeared in a number of different live action Grump moments, most notably in a Kitty Cat Dance Party at the end of Joe and Mac.
The Alcoholic: How Arin characterizes Mochi. Apparently he runs out to buy beer, even though they already have some, just to make sure they don't run out.
That Poor Cat: The loud screeching of Mimi was apparently causing the Grumps annoyance when they were meeting with their investment banker.
Doctor "Orph" Orpheus and Mojo
Ross and Holly's cats, one black and the other striped and grey. Orph's appeared in a T-shirt commercial, and been mentioned on Steam Train occasionally. Both Orph and Mojo have been featured on Meow Train, the new show on April Fools' Day.
Brought To You By The Letter "M": Mojo has a stripe on his head that resembles the letter M, and is presumably how they named him Mojo. This is even reflected in Mojo's Grump head.
Cats Are Mean: Subverted and played straight. Orph demonstrates incredible patience being played with in the T-shirt commercial outtakes...before biting Ross' hand anyway.
Also subverted, that Orph manages to be so kind that Ross is no longer afraid of cats, a justified fear since his cheek was literally torn off by a cat as a baby and had to be surgically reattached. Orph is just that friendly a cat.
Even still, Ross calls Orph a jerk for creating noise in Ross' office in the morning to wake him up to get fed.
Shrinking Violet: She seems really shy to speak on the show, and hangs up immediately after Arin and Jon spell it out that she is actually on the show. Justified in that she probably just didn't want to interrupt Jon.
Lloyd Hanson (Paparaptor)
Arin's dad. Never actually appears in the show proper, but gets brought up a number of times.
Abusive Parents: Averted. It's completely hypothetical and played for laughs. Arin tells a story about how his Dad joked he'd spank him, but assures the audience he never did and it was entirely a joke.
Repetitive Name: As mentioned in Dan's character profile, his is AviAvidan.
Retired Bad Ass: Was the commander of a tank in the Yom Kippur War, he saw some awful things. Once he was out of the military he became 'a gentle lamb' and came to America.
That Came Out Wrong: When Danny came to him about his sister beating him up, Avi said 'hit her back.' Danny went and punched her right in the back that instant. Obviously Avi didn't mean it like that.
Verbal Tic: Judging by Dan's impressions of him, he has a tendency to say "ehhhhh" between his words, either from pausing to think of what he wants to say yes (due to English not being his first language), or at least in one instance because the subject was awkward.
Confirmed when Dan lets the audience listen to a voicemail he left. His speech patterns are the same as Dan's impressions of him.
Avi: I wanted to congratulate you on breaking the Five. Million. Hits on Youtube. And ehhhhhhhhh. Enjoy them all! I'm freezing my ass off here. It never stops, ehhhhhhhhhh...snowing."
Danny explains in a later episode that the reason for the tic is because Avi's first language is Hebrew, and he translates everything to English in his head before saying it out loud; the tic occurs when he's processing it too slowly.