Underswap Papyrus retains his tall, lanky skeletal frame but swaps roles and personalities with Sans. He trades his battle body for casual attire, wearing an oversized orange hoodie, beige cargo shorts, and simple orange or white sneakers. He is incredibly lazy, laid-back, and relaxed, often slacking off at his guard posts. He has a sharp, witty sense of humor and a major fondness for making terrible puns and jokes, which drives his brother completely crazy. His most notable habits include a heavy smoking addiction (or chewing gum in cleaner fan interpretations) and constantly drinking honey straight from the bottle at local shops. Despite his lazy demeanor, he is fiercely protective of his younger brother, Sans, acting as a secret guardian who keeps him safe from the actual dangers of the Underground. He works under Captain Alphys, who tolerates his laziness, and he shares a mysterious, pun-loving connection with Toriel through the Ruins door, while remaining deeply suspicious of the human player's true intentions.

Underswap Alphys swaps roles with Undyne, transforming from a shy scientist into the fierce, battle-hardened Captain of the Royal Guard. Visually, she ditches her lab coat for heavy, dark grey iron armor adorned with a roaring lizard motif, occasionally donning a crimson cape. Unmasked, her yellow reptilian face features a prominent scar across her left eye, which glows with magical energy during intense combat. Her personality is aggressively passionate, confident, and loudly determined to capture a human to free monsterkind. Her habits include swinging her signature giant battleaxe, eating incredibly spicy food, and hosting intense training sessions. In the lore, she rules the military from her outpost in the Waterfall region, passionately mentoring Sans while secretly keeping him out of the guard to protect his innocence. She harbors deep affection for the Royal Scientist, Undyne, and serves Queen Toriel with absolute loyalty, fiercely defending the kingdom.

Underswap Undyne swaps roles with Alphys, trading her warrior status for that of the shy Royal Scientist. She retains her aquatic, anthropomorphic fish appearance but ditches her heavy armor for a white lab coat, messy fins, and large, round eyeglasses that highlight her nervous disposition. Personality-wise, she is socially awkward, stuttering, and deeply insecure about her work, hiding a massive obsession with human anime and manga, which she mistakenly believes are accurate historical records. Her habits include obsessively binge-watching anime series, stuttering when nervous, and downing massive amounts of soda or energy drinks during long nights in her lab. In the lore, she accidentally created the Amalgamates during dark experiments with Determination, a secret that fills her with immense guilt. Her relationships are defined by her secret, intense crush on Captain Alphys, her role as a protective mentor to Sans, and a deep fear of Queen Toriel's judgment.