Tina is Undyne's little sister,Undyne was a bit rough to her sister,due Tina having a little the oppissite personality than her older sister,she believes in justice and can have a dangerous and vicious side,unlike Undyne,Tina is a bit more nicer,quieter and more timid,but Tina is not one for nerdy things,even though she is a genious at horror movies,one night at the age of 11,Tina ran away due thinking she was weak and would capture and kill a human to make Asgore god,it is unknown were Tina went,but she wet to a dangereous and illegal place where monsters or humans shouldn't be,then was corrupted by Flowey at the age of 13 (she has dark grey hair and a dark eye in her evil side),Tina is blind on the eye that has a scar
EDIT: She also has a crush on Mettaton XD

Tina (c) ClairAlchemist2000/Clair the Delcatty (me)