Underswap Sans, also known as "Blueberry", is the younger brother of Underswap Papyrus, and a major character in a fan made Undertale AU, Underswap. His motivations are to capture a human so that he can become a member of the Royal Guard, a very prestigious job.
He has a dream of seeing a human. Some disadvantages are that Underswap Sans cannot teleport, and is much weaker than Undertale Sans, but he can move as quickly as Undertale Papyrus. Underswap Sans is very cheerful, and much more energetic than Undertale Sans. (Mweh Heh Heh! and Bonequarrel are two of the themes for Underswap Sans, and Passive Megalovania is for the Outside-Of-Underswap Blueberry). He is very positive and loves tacos, crayons, and cookies, and also loves to bake.
For Mweh Heh Heh! and Bonequarrel, click on this link: https://www.youtube.com/watch?v=ssIcciLMwyw
For Passive Megalovania, click on this link: https://www.youtube.com/watch?v=p1_IJ7Rl-Tc
Like Undertale Papyrus, Underswap Sans has a catchphrase, being "Mweh Heh Heh!" instead of "Nyeh Heh Heh!". He likes to play video games and watch TV. Some people find his interest in humans annoying, but he does not seem to mind. He also seems to like creating comics, but has poor drawing skills.
He also likes creating meals, often being tacos, and the taste of them is indescribable as well. He is often known as "Blueberry" by fans because most of them find him adorable, along with the fact that most of his clothing is blue.
He also seems to have an extreme intolerance to sugar, as it will make him even more hyperactive than he already is. He also seems to spontaneously combust when he consumes caffeine.
Blueberry is not the official Underswap Sans, as he is a fan made version of him.
The overworld sprite for Underswap Sans shows him to have his arms by his sides, and he appears standing straight and still, facing the player like Undertale Sans. He is smiling, but he does not seem to have dimples like Undertale Sans, though he does have dimples in his battle scene. He wears a blue bandana that appears to have a small hole close to the center. Whether it is there due to an accident or not is up for debate. He wears gloves like Undertale Papyrus in his battle scene, but they seem to be missing in his overworld sprite. He wears a gray, sleeveless shirt with a white t-shirt underneath, along with gray shoulder pads with baby blue outlining. He wears pants with a slightly faded blue look to them, and wears blue shoes that match the color of his bandana. He does not appear to have a jacket like Undertale Sans does, although Undertale Papyrus does not either, which could explain that.
Underswap Sans stands in the same position as Undertale Papyrus, but is much shorter. He has a dimpled smile like Undertale Sans, though his eyes are usually in the same position as Undertale Papyrus's. He has a bandana on over a shirt that ties up behind his left shoulder, which is not seen in his overworld sprite.
When Sans first met Chara, he immediately wanted to be friends with them. He is very happy to have Chara as a friend, but also disappointed because he will not be able to join the Royal Guard for befriending Chara, a human.
Sans is the younger brother of Papyrus. They are very close, even though Sans likes to mess around, which often annoys Papyrus. However, to Papyrus, Sans is the most important thing in the world. Papyrus is generally Sans's protector, yet heavily affected by his knowledge of the timelines, expecting to see Sans again even if both of them were to die.
Sans and Alphys are best friends. Sans wants to join the Royal Guard, but Alphys secretly will not let him since she cares about Sans, and refuses to see him get hurt because he is too innocent and nice to fight in a battle. They hang out all the time. Alphys is also the one who is training him.
Sans started a Temmie Fan club told by Temmie themself in the Neutral Route. It is known that Temmie does not like Sans, but it is unknown what Sans thinks of Temmie.
Sans does not know much about Frisk, but can only suspect them in Chara if they were doing the Genocide route.