What you are
Shadowhunter
How you meet
Before you went to the New York Institute you trained at the London Institute for a few years. You met Sebastian there and became friends. When you were 7 years old your parents took you to the New York Institute and you have lived there ever since. When Izzy brought Sebastian to the Institute you were super happy to see him again. You ran up to him and hugged him. You had to admit he grew up to be pretty hot.
First Date
Sebastian asked you if you wanted to get some ice cream/frozen yogurt with him one day because he remembered that you loved ice cream/frozen yogurt ever since you were younger. After you got your frozen treat you walked around New York for about an hour until you got a call from Alec saying you were needed for a mission.
Normal Outfit
First Kiss
You were showing Sebastian around the New York Institute. You had just finished showing him the training room and were walking towards the living quarters. You were glad that you had him back. You had shown him your room and then his room. He thanked you and you guys started to talk as you were walking down a long hallway. Suddenly you stopped and looked at him.
"Is something wrong?" Sebastian asked you
"No, nothings wrong." you replied
"Then why did you stop?" he asked, confused. You didn't say anything. Instead, you pushed him against the wall and kissed him.
Sebastian was surprised at first but he kissed back after a little bit. You only pulled away when you needed a breath.
"What was that for?" Sebastian said, smiling.
"I don't know. I guess I missed you." you told him, shrugging your shoulders.
"I missed you too." he said before pulling you in and kissing you again.
He gets jealous
Sebastian was walking through the halls of the Institute looking for you. He then heard someone playing the piano close by. He walked into the room the beautiful music was coming from and saw you and Jace playing the piano together. You were sitting side by side on the piano bench and you were playing the same song but in different octaves. When Sebastian saw this he couldn't help but get jealous. You were smiling with Jace and you were both enjoying the moment together. Sebastian cleared his throat loudly to get your attention. You and Jace both stopped playing and looked at Sebastian.
"Hey Sebastian" Jace said nervously, due to the glare he was receiving from Sebastian. You noticed the awkward tension and quickly got up from the bench and went over to Sebastian. You pulled Sebastian out of the room while looking back over your shoulder, mouthing a "sorry" to Jace.
"What was with the glare you were giving Jace?" you asked Sebastian when you were far enough away from the room that you were sure Jace wouldn't hear you. Sebastian didn't say anything.
"Sebastian" you said a little sternly.
"He was sitting too close to you and he was making you smile and he probably has feelings for you." Sebastian replied angrily.
"I know he has feelings for me. Feelings of friendship and nothing more." you tried to explain.
"It didn't look like just feelings of friendship" he mumbled
"Aww is someone a little jealous?" you teased him, poking his shoulder
"Maybe I am but I have every right to be" he says.
"You have the right to be jealous, but you shouldn't be jealous." you told him
"And why is that?" he asked
"Because I would never do this to Jace." you said before pulling Sebastian down to kiss him. You wrapped your arms around his neck while he wrapped his around your waist. You pulled away and rested your forehead against his.
"Don't be jealous, okay?" you whispered
"Okay." he whispered back, then kissed you again.
Who he is jealous of
Sebastian was jealous of Alec at first. Alec is your best friend so you hang out with him a lot. When you found out about Sebastian's jealousy towards Alec you explained that Alec already has a boyfriend. Sebastian got a bit embarrassed until you reassured him that you thought it was cute. After he found out that your parents were friends with the Herondales and that Jace is a Herondale, Sebastian became jealous of Jace because you started to hang out with Jace more often.
Who you are jealous of
You get jealous of Izzy because Sebastian helps her kick her Yin Fen addiction. They are really close because of it but you know that Izzy would never betray you like that. You are also jealous of Clary because Sebastian has seemed to take a liking to her and you have never trusted her. You always make sure to be doing some PDA with Sebastian when Clary is around.
Who you are related to
You are an only child. Your parents were friends with the Herondales so you are respected a little more than some of your friends and fellow shadowhunters.
Who your best friend is
Alec Lightwood is your best friend. You and him just clicked when you came to the New York Institute. He is like the brother you never had and you are like another younger sister to him.
Who your enemy is
Being a Shadowhunter you naturally had lots of enemies. As a Shadowhunter your biggest enemy was demons or Valentine but as a human your biggest enemy was probably Clary. You thought that she was self absorbed and she always thought she was better than everyone. You also hated her because it seemed like Sebastian had a thing for her. Your life was so much better before that red headed "Shadowhunter wannabe" came along and began to destroy the Shadow world.
Work Outfit
His favourite thing about you (personality)
Sebastian loves when you show off your intelligence to people. When you become a know-it-all Sebastian can't help but smile. You know so much about the Shadow world but you also know a lot about the mundane world. You love to just go out in the mundane world with your invisibility rune activated so you can just learn more about the world you never got a chance to live in.
His favourite thing about you (facial)
Sebastian loves your smile. He loves when you are happy. He loves it even more when he is the reason you are smiling. He loves how your eyes sparkle when you smile and he just can't get over how beautiful you look when you smile.
His favourite thing about you (Physical)
Sebastian loves your stomach. He loves to kiss your stomach because you laugh when he does. He also loves to tickle your stomach. You are sometimes insecure about your stomach especially when you feel bloated (you usually feel this way when you're on your period or have eaten a lot of food *food baby*), but Sebastian always has a way of making you feel better.
First Fight
You had been told to watch Valentine through the surveillance cameras. He knew someone was watching him through the cameras so he taunted them. He kept saying that he was going to get out of his cell and kill everyone. You ignored it at first but when he started bringing up your friends names, you started to get pissed off. You went down to his cell and started to threaten him through the glass.
"I swear to the angel if you say one more thing about my friends I will go in there and I will make you pay for everything you have done." You say to him.
"Oh so you don't want to hear me talk about what I plan to do to Jace, or either of the Lightwood children?" Valentine said.
"That's it." You said as you opened the door to the cell and walked in. You pulled your Seraph blade out and swung it at Valentine. He dodged it a few times before you pinned him to the wall with your blade against his throat.
"Go ahead kill me. You will just have to face the punishment of the Clave when they find out." He smiles at you. Just as you are about to cut Valentine's throat, Sebastian runs in and grabs the blade out of your hand. He takes you out of the cell and locks the door back up. You walk to the elevator without a word and Sebastian soon joins you in the elevator.
"What the hell were you thinking y/n?" Sebastian yells at you.
"Valentine deserves to die. He has killed thousands of innocent people and yet the Clave still lets him live, all because they think that they can get him to tell them where the stupid cup is!" You screamed back at him.
"It is not your responsibility to make that call. I understand that he has done terrible things but you should not have done that and put yourself in danger!" He yells at you.
"I don't give a fuck if I put myself in danger!" The elevator had stopped at the main floor and you walk out still yelling. Everyone in the main area of the Institute looks at you and Sebastian and you continue to fight.
"I am sick and tired of hearing him talk about what he is going to do when he gets out of that cell!" You yelled not caring about all the stares you were getting from the other Shadowhunters. Alec and Jace had heard and saw and walked over to try to calm you down.
"Y/n, what is going on? What's wrong?" Jace asked you, concerned.
"This has nothing to do with you Jace, how about you leave my girlfriend alone!" Sebastian yelled at Jace. Sebastian had just crossed the line. Nobody talks to your friends and family like that. You brought your hand up and smacked Sebastian right across the face. You pointed your finger at him.
"Don't fucking talk to him like that! He is just concerned about me!" You yelled before storming off into your bedroom and locking the door.
Time Skip
It had been a few hours since you stormed off into your room. Jace and Alec knew that they should just let you cool down for a bit before trying to talk to you, but Sebastian did not know. He had tried to talk to you about 100 times before giving up when you had ignored him or told him to "fuck off".
You had taken a shower and got changed into some comfortable clothes when you heard a soft knock on your door. You opened the door to see Jace and Alec. You invited them in and they asked you what happened. You told them the entire story starting with Valentine threatening them to you storming off into your room. Jace and Alec told you that they understood why you did what you did but they also told you that you shouldn't have done that.
"You could have gotten yourself killed y/n." Alec said softly.
"I didn't think about that. I only thought about you guys. I hated hearing him threaten you guys." Tears started to form in your eyes but you held them back.
"Y/n, if anything had happened to you we wouldn't know what to do with ourselves. We love you so much and we couldn't live without you." Jace explained to you. You nodded and hugged them both. Suddenly you heard another soft knock at your door. You got up and opened the door. Sebastian was standing there with a sad look on his face.
"We will leave the two of you alone." Alec said as Jace and him left the room. Neither of you said anything for a while, you both just stood there and looked at each other until he pulled you into a hug. You hugged him back like your life depended on it.
"I'm so sorry for yelling at you." Sebastian quickly apologized while still holding you in his embrace.
"No, I'm sorry. You were right to yell at me for doing something so stupid and reckless. I was only thinking about what was happening in that moment. I never thought about what would happen if it went wrong." You cried into your boyfriend's chest.
"I only yelled at you because I was worried about you. I don't know what I would do if something happened to you. And I'm sorry for saying those things to Jace. That was way out of line and I regret every single word I said." Sebastian said to you.
"Well, I'm sorry for smacking you." You said, pulling away and putting your hand gently on his cheek where you smacked him a few hours ago.
"It's okay. I deserved it." He laughed a little. A small smile made its way onto your face as Sebastian took you over to your bed and laid you down. He climbed into the bed beside you and wrapped his arms around you until you fell asleep.
I hope you guys liked it. Requests are still open. If you want to request an imagine or preference just private message me and I will try to get it done asap.