Love interest question. Do you want Mordred to be the only love interest or should someone else join in? Don't be shy! I'll think over every proposition!
If you enjoy my OC work, be sure to check out my OC journal I made. ( I'll put link in comments for easy access. There's a Predator based Servant there already. )
( Narrator P.o.V )
Chaldea
Summoning Room
Young master, Ritsuka Fujimaru, managed to get back from being pulled into her first singularity. Mashu, now a Demi-Servant, and Roman, doctor from Chaldea, had to put the weight of humankind on her shoulders. She felt it weight her down and she knew that she has to go all out or the spark of humanity will die down completely.
Mashu explained to Ritsuka how the Saint Quartz works and brought her to the summoning chamber. She has to try summoning her first Servant and she feels like it's a great responsibility to get the strongest there is to aid them in their goal.
She sets down the Saint Quartz in it's places before taking a few deep breaths and - doing much like Mashu has explained - started to open the flow of magic on the cristals while pouring her power on it. She felt the magical circuits in her body flaring to life but she couldn't stop the overwhelming feeling of hundreds of eyes resting upon her.
Ritsuka : Please... I need strength to save humanity...
She poured even more magic into it, not understanding that she's overloading her own magic circuits which may damage the summoning and completely destroy the Quartz. Suddenly the Quartz flared with light before rising and spinning in circle. Light filled the summoning chamber as a Servant was being pulled throught...
Ritsuka cut off the stream of magic and fell to her knees from exhaustion as a figure started to form. Her eyes widen as the person appeared. Tall knight in armor and giant axe at his side stood right in front of her. She felt kind of small in his presence...
(Y/N) : I am (Y/N) the Kinslayer! Avenger class Servant! ...Is Mordred around by any chance?
Ritsuka : I'm your Master, Ritsuka Fujimaru! Sorry, but you're my first summoned Servant - aside from Caster Cu.
(Y/N) : No need to beat yourself up over this, Master. I'm sure she'll come around eventually, she just loves getting into troubles.
Ritsuka : ...She?
(Y/N) : Well, not many know about it, I guess...
Ritsuka : You'll be able to tell me all about this! For now how about you'll meet others?
(Y/N) : Lead the way!
She was very happy! She just managed to summon a powerful Servant, five stars and quite a firepower! He was also an Avenger but she didn't quite get it... She felt like she really needs to catch up on all possible Servant classes. She took him on a small tour showing the Servant dorms, that at the moment are completely empty, and pointed out that he can take a room.
(Y/N) : Hmmm...
Ritsuka : Is something the matter?
(Y/N) : Do you have any spare beds?
Ritsuka : I guess there are a few in case something happened. They're in the storage. Why?
(Y/N) : I like bigger beds, besides once you'll summon Mordred we're gonna room together.
Ritsuka : Are you and Mordred...?
(Y/N) : Lovers? Yes... We confessed right before the Battle of Camlann.
Ritsuka : It's kind of sweet actually.
(Y/N) : Don't let Mordred hear that, she's gonna get all pouty.
As the two shared a laugh something else has arrived at the room. Fou looked over the freshly summoned Servant before going over to them to say hi. (Y/N) couldn't help but take of his armored glove and pet the creature.
Afterwards she decided to show the rest of Chaldea, including places such as cafeteria - the most important place that all of the Saberface family will siege - but lacking a cook, a few rooms where Servants and Masters could spend playing multitude of games and restrooms - with hot springs that are divided for male and female Servants / Masters but I'm pretty sure SOME Servants wouldn't care about that.
Of course one of the important stops was Da Vinic's workshop from which Ritsuka could enhance her newly summoned Servant with some cards.
Leonardo : Hello Ritsuka, is this the new Servant?
Ritsuka : His name is (Y/N) and he's an Avenger!
Suddenly there was a commotion as they turn towards door seeing Roman there.
Roman : I heard that Ritsuka took new Servant for a stroll! Woah!
Leonardo : Knight armed with axe and Avenger class... (Y/N), then you must be the Kinslayer.
Roman : Ah yes, the knight who once was apart of the Round Table and King Arthur's loyal subject. He joined Mordred since they were the best friends - or since Mordred is female we can assume lovers - and fought and died during the Battle of Camlann. He has slain many knights who followed Arthur into battle before being struck down with holy lance.
(Y/N) : My late life in a pill. Legend of Kinslayer, killer who murdered knights of the Round Table and got a justful death by Arthur, or rather Artoria's, hand who died afterwards from wounds sustained in battle with Mordred who also died... That's a lot of death.
Mash : ...
(Y/N) : Agh, when did you get here!?
Mash : Oh, I just waited for you to finish talking so I can introduce myself. Please, forgive me. I didn't mean to surprise you.
(Y/N) : It's quite fine...
After everyone was done introducing themselves, with Leonardo doing big deal out of the fact that despite being a Servant (Y/N) don't know about her, Ritsuka took him to enhance him a bit. She threw all EXP cards she had on him to give him more firepower. Now being at 30 level he had far more damage output and could take some more punishment.
(Y/N) : More strength to serve! Let me at them, Master!
Ritsuka : I'm happy you're so excited! Roman said about locating new singularity so I need you combat-ready!
(Y/N) : You can depend on me, Master! ...But I think you should rest. Summoning me took quite a toll on you.
Ritsuka : It's okay, nothing serious!
(Y/N) : You're as stubborn as Mordred... No talking back.
He suddenly walked over to Ritsuka and lift her up bridal style. Despite all of Ritsuka's protests he just kept on walking and ignoring her pouting and yelling. Upon entering the Master part of dorms he has noticed Mash standing on the side looking around... Probably trying to find Fou.
Mash : Senpai...? (Y/N)...?
Ritsuka : It's really not what you think!
(Y/N) : Mash, where is Master's bedroom?
Mash : It's at the end of the hall. Right door.
(Y/N) : Thank you. I own you one so don't be afraid to come over if you need anything.
As the two started to walk away - or rather (Y/N) carried pouting Ritsuka - Mash just scratched her cheek and looked after the two. Commotion causing for Cu to walk towards the Masters' dorm and noticing Mash looking after the pair.
Mash : Senpai and (Y/N)...
Cu : They're quite bold, aren't they?
Mash : Hmmm, (Y/N) is returning.
Cu : That was quick.
Mash : What happened?
(Y/N) : I put Master in bed and told her to rest. If I find her outside her room without her resting I will drag her there again and stay there until she gets some rest.
Cu : You seem strangely accustomed to it.
(Y/N) : As much as I love Mordred she acts like a kid sometimes... Someone had to be responsible for her well-being.
As the trio was talking Roman was walking down a corridor, obviously heading for Ritsuka's room but an armored hand grabbed him by the scruff of his coat and dragged back.
(Y/N) : Master must rest. Do not disturb her.
Roman : Oh, she finally decided to rest after being done with Singularity? Just inform her that I'm done with finding the new Singularity.
(Y/N) : Where is it?
Roman : You're going to France!