Expressive Mask: Most noticeable on Sniper, especially after the Biomatrix integrates it into his body.
The Faceless: Only two of the main four have been seen without gas masks on, and even then their faces were always partially obscured by something else.
Gas Mask, Longcoat: Only the Captain has the longcoat part of the equation, but they all have the gas mask.
In-Series Nickname: Each character is most often known by their nicknames. And while Sniper, Pilot, and Engi have each been revealed to have full names, Zee Captain is still only known as that (or simply Captain, or Capetian.
Known Only by Their Nickname: Again, subverted with Mr. Snippy. Snippy also gives his full name to the Captain in the beginning. All but the Captain's full names are known in-universe, if not necessarily by the other main characters.
Vague Age: One of their many mysteries. All we really know (warning, video contains spoilers) is that (excluding Captain) Engie is the oldest, Snippy is the second oldest, and Pilot is the youngest seeing as how he was made in a factory and all.
The Captain (a.k.a. "Zee Captein")
Born Lucky: And according to Engineer, it may be contagious.
The Captain: Well, yeah. He/She was originally known as Subject Seven, but refused to respond to anything other than "Zee Captain" after attaining that rank.
Heavy Sleeper: It's not uncommon for a strip to begin with Snippy waking up and realizing that either Captain or Pilot have vandalized his body in some way while he slept. He also mentions averaging about nine hours of sleep a night, which is apparently unusual for the others (Captain mentions doing activities with Pilot that are past Snippy's "bed time").
Snippy's sleeping habits make perfect sense considering that he was unable to afford sleep in the pre-apocalyptic world.
Fun with Acronyms: "Pi" refers both to his job as a private investigator and the mathematical constant.
Hipster: Back when he was still working for the Director, Pilot insisted on modeling his office after the classic Film Noir style detective rooms, complete with handheld phone. Both of these things are hilariously outdated for the times. Justified in that cutting down on the advanced technology in his life makes him feel human.
Loving a Shadow: Thinks Zee Captain to be a god, though Zee Captain is implied to not reciprocate (responding with flat dismissal whenever Pilot makes advances through the blogs, and having said that his "minions" are "all equally tolerable").
Man Child: Pilot's immaturity levels rival Captain's.
No Name Given: A name is written on his jacket, but he doesn't think it's his. However, the Biomass calling him by it and his wearing the jacket in a pre-apocalypse flashback imply that it might be, and he is just too far gone to remember.
What Is This Thing You Call Love?: Pilot mentions in passing how "they" told him that he's not capable of ever truly being in love, which is almost certainly due to him actually being a robot of some kind. He claims that it isn't true.
Trauma-Induced Amnesia: His amnesia and insanity were caused by a grenade blowing up in his face, severing his connection to ANNET.
Yandere: Of the Type 2 "Obsessive" variety towards Captain, who he treats like a god.
The Engineer (a.k.a. "Engie", a.k.a. Dr. Alexander Gromov)
Foe Yay: ANNET called him "dear" and said "I love you" whilst trying to kill him years ago. In her return she referred to him as "my love", just before trying to kill him. Likewise, Engineer usually calls her "Annie", and before the apocalypse it was definitely a term of affection. He also has called her "my love." He even wrote a poem about her, in which he called her "my beloved. invoked
Hypochondria: He suffers from this. It's especially notable in issue 172, where he is almost certainly in mortal peril yet still chooses to focus on not letting a blood covered sword touch him.
Only Sane Man: Type 3. Unlike Snippy, he's completely given up and doesn't even bother protesting the strangeness of his companions. Although one of Snippy's blog entries implies that he might not be all that sane after all.
Evolutionary Levels: The Cancer incorrectly refers to the Lifealope's shapeshifting ability as "evolving". In keeping with the trope, it seems to become more intelligent and complex with each shapeshift.
No Name Given: Unlike the other characters, it doesn't even have an In-Series Nickname. The closest thing is a wallpaper of it titled "WOLFALOPE." To compensate, fans have dubbed it "the Lifalope", a portmanteau of "life" (because of its Fertile Feet) and "wolfalope."
Ascended Fanon: The character was eventually listed as "Lifalope" on the info page of the official RA website.
Shapeshifting: Started out as something resembling a salamander or lizard, then shapeshifted into a rabbit, then a fox/wolf-like creature, and then seemed to resemble a moose or deer before being killed by ANNET's ion cannon. It was reborn as a cat-like creature (a wallpaper by the creator calls it "CATALOPE")]]. The only features that have remained constant are its three eyes and the presence of antlers.