W.D. Gaster

Gaster, full name W.D. Gaster, is the father of Sans and Papyrus and the adoptive father of Shae. He is Ariel's romantic partner as their marriage in the past was voided when he was believed to be dead. He was the Royal Scientist before Alphys and is the creator of the CORE.

One day, he fell into the CORE after being shoved in by Jerry. For years he has been trapped in the VOID (described by one of his followers that he was "shattered across space and time") and was eventually saved by Shae. He reunites with his family and stays with them to enrich their lives with memes and science.

Pre-CORE Incident
Gaster was a somewhat lanky and tall skeleton with a rounded skull and wore glasses. He wore a black long coat and black pants, along with a turtleneck. He often wears his lab coat in place of his long coat. He wears dress shoes.

Post-CORE Incident
After he fell into the CORE, Gaster's physical form changed drastically. He lost his legs and nose entirely and the rest of his body turned into a goop-like unshaped 'blob', save for his skull. He gains face scars; a line trails up his right eye socket, while another travels down below his left eye socket. Both of his hands now have large holes in the palms. He still has the turtleneck and his glasses. His skull became more rounded with no definition in it.

Pre-CORE Incident
According to Ariel, Gaster was a shy, awkward, and nerdy skeleton. He had a soft spot for humans (and still does). He did not believe in doing cruel experiments. He was also very fatherly towards both his sons.

Post-CORE Incident
The incident drastically changes Gaster's personality and his mentality; he is less shy and awkward and now more comedically snarky and sometimes can come off as apathetic. He is still fatherly, but often shows it in a memeful way. He often does pranks and is absolutely obsessed with memes to the point that he is described by his children to be "A living meme". He litters his speech with text slang, specifically "lol", "lmao", and "wtf". He also may spell you as "U" and right as "rite".

When he is enraged or trying to appear mysterious, his pupils will appear.

Abilities
Gaster has all standard monster abilities and skeleton monster abilities, including ones exclusive to the skelefam.

However, his magic is much more powerful, as he has the power of the void enhancing his magic, though he was already powerful before he had gotten trapped in the void.

Trivia

 * Gaster's most recurring gag is the popular meme "It's nerf or nothing!"
 * Gaster's tendency to say "U rite" and spell it as such is a reference to YouTuber Markiplier, who does the same as a gag in his videos.
 * Many magic abilities are exclusive to the skelefam because of Gaster researching and enhancing his magic beyond the normal skeleton monster magic standards (such as telekinesis and SOUL mode blue), and teaching it to the whole family, including honorary Ariel.