Sellia is a character in Underscratch. She is part of the Royal Guard.
Sellia is a main character, but she does not replace Undyne, because she barely feels determined. Anyways, at first, she takes you calmly, but if you annoy her with your actions, or do a Genocide run, she`ll go hostile on you. She`ll fight you if she`s calm, anyway.
If you have not killed anybody, she will say that you may have some unfinished business (Hinting that you may have not befriended Papyrus). If you have killed anyone, she will say "Why are you killing friendly monsters...?" if you answer "Kill or be killed", she will say "That`s a myth. Who told you that?" if you answer "Flowey the Flower", then she`ll say "Flowey...? The-" And then gets interrupted by Flowey, and Flowey will say "IDIOT". Then he`ll kill Sellia, however this backfires, as Sellia blocks the attack, and Flowey escapes. If you answer "A dog", a small dog will come and "Dogsong" will play. Then the battle with Sellia will be interrupted for some time. If you have killed Papyrus, she will be worried about him, and assuming you killed him.
If you befriended Papyrus, she will say that you have made quite the friends. She will challenge you to a duel, saying that if you defeat her by killing her, she`ll give you all her gold, but if you spare her, you`ll only get 400 gold. She carries 1600 gold. If you spare her, she will be surprised that you spared her, and decides to give you a "Plush Sword", which can be equipped to gain a few ATTK. She says that she dueled you because she has "nothing to do". You can befriend her at the MTT Resort after Sans invites you.
And then she`ll get annoyed. She will say if you keep this up, Undyne will strike you down. Then you`ll have to fight her. Sparing heGenocide: She will attempt to kill you brutally. If you accept her mercy, she`ll say you gave up for a good reason, and the route will change to Neutral. If you kill her, she`ll say that Undyne will strike you down, but if she offers mercy, and you kill her, she will say you are worse than the other 7 humans.
Sparing her will cause her to say "So... That`s good. The demon inside has been cleaned!". If you kill her, she will say that you are "one who is possessed". She will say this only in the final battle.
Story (Outside Battle):
She usually is friends with people who do not kill, but if you killed somebody before, she will still be your friend, but be a little bit disgusted at you. You can be her little buddy at Waterfall, but you cannot find her in the MTT Resort or Grillby`s if you killed 1 monster. If you killed all of her friends (except Red, she is encountered much later), she will be even more disgusted at you.
She will give you a choice to go to Grillby`s or MTT Resort. If you choose Grillby`s, she will be happy, since her friends hang there a lot. She will then order you a burger and some fries. She also will break out a "Royal Meeting" about who is doing fine, what`s happening, and latest news.
If you choose MTT Resort, she will invite her friends (Doggo, Dogamy, Dogressa, Lesser Dog, Greater Dog, Red, and Undyne). After she invites all of them, Red will question about what this place is, since she never been in MTT Resort. Nobody answers her question (Except Greater and Lesser Dog, but they don`t speak), so she gets furious. Sellia then says that the meal is on her. She gets you dog salad, Starfait, and a "Dog Pie". Doggo questions why Sellia is inviting them over here, and she answers by saying "Well, this human said that we should try something new..." and then everyone gets frozen for some seconds, but then everyone gets happy and thanks Sellia for the meal. Sellia will then invite everyone to her house, and wear her "pajamas, since it is a sleepover, and everyone else does, too (Except Red, since she takes Night Duty) . You then need to wait a few seconds, and then everyone will wake up. Sellia will then ask how you are doing.
She will not invite you to be friends with her. She says that if you get rid of the demon inside of you, she MAY be friends with you. If you kill her, she will say that you are "the demon who calls it`s name" and die.