W._D._Gaster/Handplates was the royal scientist before Alphys and the creator of Sans and Papyrus. It is heavily implied that he is one of very few (if not only) skeletons that survived the war as he states at multiple points that there are little examples for what is normal for a skeleton and reacts fondly to rattling bones because he rarely hears it.
The holes in his hands were cut by him so he could use the material to create the brothers. He uses Sans and Papyrus for experimentation but the end goal is currently unknown (though it is safe to assume it somehow links with breaking of the barrier). He has killed before though the circumstances under which he did so is unknown and has done research on the effect of killing on a beings soul both figurative and literal.
W.D. is usually depicted as a lost soul with the white effect covering his face. He wears a labcoat and dark grey under shirt. He has holes cut into his hands that were not natural to him.
When not a lost soul, he appears as an ordinary humanoid skeleton (sometimes with a short tail) without any cracks in his skull and wearing a pair of glasses that are taped to his head. Despite a lack of a visible crack, one of his eyes is damaged though what the nature of the injury and what caused it is unknown. As a child he is shown with a scarf similar to Papyrus's and he may be wearing the same hoodie as Sans.
Later in the series he gains the cracks but we currently have not seem him without the lost soul effect.
Although he goes through with increadibly unethical experiments, he maintains a strong friendship with Asgore (the saving of whom seems to be his main motivation).
Alphys also cares deeply for Gaster and show clear concern for him. They have been seen watching anime together. She has a crush on him (because of cause she does).
Prior to leaving the castle, Toriel was also caring towards him.
He tries to maintain a cold and heartless mentailty with the brothers though he occasionally has moments of weakness.
- Papyrus's refusal to kill is seen as something to be challenged. He even threatens to murder Sans to see if Papyrus will kill to save him but the threat is a bluff (whether or not Papyrus does kill is left up to the reader).
- Hearing the brothers rattle their bones provides him both comfort and entertainment.
- He has a fondness for marshmallows.
- The cracks in W. D.'s skull was caused by an incident with the force field grids that prevents the brothers from leaving their cell.
- he smokes