Tasin Sans is the only survivor of the war between humans and monsters in which the humans killed monsters instead of sealing them underground. Tasin Sans lives alone on top of Mt. Ebott and has very little knowledge on the happenings in the other AUs or who Frisk and Chara are because monsters were never trapped so they've never met.
Tasin San's only goal is to find Dragon Sans knowing that he had the ability to open portals to the other AUs. This is what has kept Tasin sane all these years.
Tasin lives in the genocide timeline which took place during the first war with monsters and humans. Humans managed to kill all the monsters leaving Sans as the last one.
San's desire is to find his bonded partner and to finally take vengeance on the humans that killed all his friends and family, this is why he first meets Ink!Sans. Tasin puts his best mask on and wins him over with a cute and innocent act so as to be taken to other AU's in search of Alise(Wot) and his bonded partner.
Tasin would usually change personalities from cute and innocent to a more menacing side as Dragon is Tasin's other half. Most would say that Tasin is incomplete without him and becomes instable. It's Dragon's job to tell Tasin what is wrong and right, and what to do in certain situations. This is why when Tasin gets into a fight he would automatically change and charge in head first. Without Dragon, he becomes mindless monster who only knows how to fight and can't think for himself.
Tasin has three deep scars in his left cheek bone and a large crack on the left side of his skull that reaches his left eye. He also has three marks on each side of his face resembling six dark blue triangles which represent protection and a dark blue line that goes down from his eyes to resemble the blood of monsters. Tasin wears a white version of UT Sans' Jacket with a black scarf and red shirt with black shorts and black slippers.</p>
Abilities and Weaknesses
Tasin is able to change his form into that of a skeletal dragon and gains the ability to fly and breath blue fire. In doing so, he loses all focus of the real world and loses control when attacking any and all obstacles in his way. The only way to defeat him is when he is in this state is, with Dragon or Knowledge Sans in order to fight and defeat him.
Dragon is the bonded partner of Tasin Sans who would always explain everything to Tasin. He is the only one who is strong enough to fight Tasin and win.
Dragon is the opposite of Tasin. He is able to open portals to other AUs. This is why Tasin is so insistent on checking them in hope that Dragon survived and is in another AU.
They are brothers but not brothers. They are also lovers even though they are bonded, it's complicated.
Ink was the first Sans to find Tasin and taught him new things that weren't about Dragon. It's because of this that Tasin treats Ink with respect and follows him around. When he is with Ink he acts cute and innocent in order to get answers and earn his trust.
Ink lets his guard down and trusts him.
Tasin Sans treats Fresh mostly like a brother and would be found imitating him. He would do anything that Fresh asked in a heartbeat if it meant he would make him happy.
Error has mixed feelings about Tasin. On one hand he is another abomination and every time his personality switches to a more dark side then Tasin would quickly switch back to cute making it hard for Error to destroy him.
Death has meet Tasin during the war and therefore knows all of his secreats. Death would usually call him Darkness quoting something that Dragon once told him about the name Tasin meaning both Protection and Destruction.
Tasin only met Geno once when he was with Reaper Sans journeying. During that time Geno gained mixed feelings for him seeing both his cute and darker side. After their meeting, Reaper told him not to go near Tasin saying that he was too unstable to be around when Dragon is not present.
Tasin about Nightmare: 'Thanks to him, I haven't forgotten the past and one day I can turn that pain into strength and Nightmare has given me a reason to live.'
When Nightmare first meets Tasin in person, Tasin thanked him for all ways he gave him dreams of the war and Dragon. Because of this, Nightmare treats Tasin better then anyone else, going so far as to give him a nightmare apple from the tree and thinks of Tasin as his only true fiend.
Dream has mixed feelings for Tasin, being able to feel his pain and suffering and knows about his hope of finding Dragon.
BlueBerry is afraid of Tasin because he witnessed Tasin's darkness and transformation. When Tasin saw Chara, he attempted to kill her, losing all rationality and relied on instinct. He couldn't see that he got the Chara of BlueBerry's and his universe mixed up.
Although the monsters were never sealed underground, Tasin and Dragon did meet Chara. Chara was the human who killed their family and left without ever revealing their name.