Sans

Sans is a recruitable secret character in Disgaea: Shattering Reflections is a major character in Undertale. He is unlocked by passing the Rebellion Assembly Bill Watch Some Punny Stand-up! for 7000 Mana and completing the associated Map.''

Story
Sans is a major character in Undertale. An ally of the Ruins caretaker, he watches the main character and judges their actions as they explore the Underground. While he comes off lazy, he knows more than he lets on. His backstory is a complete mystery.

Disgaea: Shattered Reflections
The Dimension Guide notes the presence of a foreign figure in the Dinner Theatre. Upon arrival, Maxwell and company are just in time to witness a hoodie-wearing skeleton performing a comedy routine. After trying and failing to impress the patrons with his eternally growing list of puns, the patrons boo him off of the stage and attempt to eat him. In a flashy display, the skeleton utilizes flying weapons and completely obliterates the present patrons. The skeleton discovers the party, and, assuming they're also hungry, attacks them. After being defeated, the skeleton realizes they are non-hostile and stands down. He addresses himself as Sans, and explains that while tinkering with an invention, a malfunction opened up a wormhole that led him into the Netherworlds. After asking Maxwell a few questions, he says that he reminds him of "a certain child." Without any other place to go, he joins the Liberation League.

Upon reaching maximum approval with Sans, he opens a portal to The Underground, an area of the Human World which functions much like a Netherworld. Here, he explains that the Underground is subject to a unique distortion in which people with certain abilities can reset and/or restructure events that occur within the Underground. As such, Sans has made it his mission to look into this "Determination," and if possible, disable those that use it to harm. He's convinced that this version of the Underground has one such person, and realizing Maxwell's strength, hopes he can help locate it. As they travel, they come across Sans' brother Papyrus, who decides to subject Maxwell to a Geo puzzle.

After the puzzle is solved, the duo, accompanied by Papyrus, discover a wounded Undyne. She says that something has "taken over" Frisk, and is attacking Snowdin. Recognizing this as their bounty, the two leave Papyrus to look after her as they proceed to Snowdin. They discover Frisk, who has been possessed by the Determination-user's demonic aura. Sans identifies this being as Chara, the First Human, who wanders the Underground timelines seeking to destroy them. Forced into battle, they defeat Chara and Frisk. Frisk escapes from his control, and the duo battle Chara one final time. Upon defeat, she pledges to return and leaves the timeline, leaving behind the Heart Locket Emblem. Recognizing Maxwell's strength and kind heart, Sans begins addressing Maxwell as "bro," something he usually only reserves for Papyrus. Papyrus and Undyne vow to continue safeguarding the Underground, and Frisk has begun a community service campaign to repair the damage caused by Chara. Sans chooses to stick with Maxwell a bit longer, and leave to the Liberation League HQ along with him.

Personality
Sans is almost fully comprised of hidden depths. Outwardly, he appears as lazy, lethargic, and playful. In reality, he has seen a lot, and this is reflected in his combat experience. He has an almost nihilistic view of the world and as such takes even the worst-case scenarios in stride. Despite this though, he has a kind heart and only wants what's best for those around him.....though getting him to admit this is an exercise in futility.