This is FGO based. That being said, there might be spoilers in here somewhere. I think...maybe
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        The past is the past. For servants, it's the legacy that gave them the qualifications of becoming heroic spirits. Often times, the qualifications are bypassed, bent, or downright ignored as far as Chaldean servants were concerned, but Mordred was normal in that regard. Her past was probably seen as a tragic existence to some, and to others a tale of valor and glory should they choose to view it that way. To Mordred, it was really nothing. Her only two regrets were that she was unable to be acknowledged by the king and that she had been unable to save the life of her only true friend.

        It had been a normal day in Camelot, so the knights hadn't expected any troubles later that evening. It wasn't that they weren't ready for it, they just didn't expect the large army to attack at random. Camelot wasn't the type of place an enemy just raided on a whim. "You could have stayed by my side and let me help you. I'm sure we could have beaten that damned thing without you losing your life...(y/n)." Mordred frowned at the thought.

        The army hadn't really been the problem. Merlin's nemesis, the mage called Mab, had sent armor clad shadows into battle knowing that they wouldn't hold long against the knights of the round. Another danger posed a threat to the people, and it was (y/n) who noticed it before the beast could reach Camelot.

        One could have called it an oversized serpent if they wished. Alone, (y/n) stood no chance against the beast that towered so far above Camelot that it nearly touched the clouds. King Arther herself would have had a hard time defeating the venom spiting serpent, but (y/n) achieved the impossible.

        Not only had he struck down the enemy with the power bestowed on him by the curse he was born with, but he also saved the city of Camelot itself. Not a single life other than his own was lost that night. What made it worse waa that Mordred had been one of the few to mour the lost of the knight who served whoever he wanted to rather than the king.

        "Mordred...is something bothering you?" Ristuka asked her. She was crashing in his room for the time being simply because she didn't feel like being bothered by the other servants. Ritsuka and Fran were there playing checkers while she had been staring boredly at the ceiling with Fou sleeping in her lap. "You seem kind of upset about whatever's on your mind." Fran nodded in agreement with Ritsuka's statement.

        "Me? Upset? Nothing's bothering me. I'm just bored as hell," she muttered. Of course, she was only telling half the truth, but Ritsuka seemed to buy her words and left her alone. For the next thirty minutes or so, Mordred let her mind wander to her own past again.

        "You idiot...going and dying on your own like that," she thought in bitter reminiscence. (Y/n), the knight that the king never acknowledged as one of the round, was truly a hero of the people despite his standing as a contradictory knight. He had been one that didn't hesitate to do whatever was on his mind, and surprisingly, that wasn't what got him killed. For the one time in his life, he had willingly followed the orders of the king rather than following his own set of rules.

        A knock at the door caught everyone's attention. "Come in. Door's unlocked," Ritsuka answered. Yes, the Chaldean master had to lock his door every now and then because of servants like Tamamoe and Kiyohime who liked to hide in his room to potentially ambush him at night.

        "Is this a bad time to ask for a favor? I found this and I thought Master would be able to use it." Nursery Rhyme had appeared with a summon ticket in hand and proudly walked over to Ritsuka to deliver the small treasure.

        "Thank you. Do you want me to use it now? I can't promise you'll get any new friends," Ritsuka spoke. He saw right past the girl's intentions even if she meant what she said. It was true that Nursery Rhyme wanted to give Ritsuka a gift, but she also knew that it could possibly bring a new heroic spirit to Chaldea. In response to Ritsuka's question, Nursery Rhyme shyly nodded. "Alright then let's go. Mordred, Fran, are you two going to come too?"

        "I'll pass. I don't feel like moving," Mordred answered almost immediately. For whatever reason, she chose to keep her thoughts in the past, thinking only of the one she regreted losing.

        "He promised to die after me...to get revenge on whoever put me to death." Mordred sighed at the thought. Many times Mordred would find herself feeling like a normal citizen of Camelot rather than one of its main protectors whenever she escaped her duties by sparring with (y/n). Silence flooded the room after she let her mind stop taking visits to the past. It wasn't until Mordred was about to get up and search for Ritsuka, that she was presented with a shock she never expected.

        "Mordred, you'll never believe this! Nursery Rhyme's ticket brought a new servant. A new spirit origin got registered," Ritsuka announced as he opened the door with enthusiasm. Mordred hadn't heard a word he said. Her gaze had gone straight past him and Nursery Rhyme. Even Fran was invisible to her as she stared the newcomer right in his familiar eyes. "Mordred? You okay?" Ritsuka inquired.

        "This can't be real," Mordred thought. "My mind is making this up right now." But that wasn't the case. It was indeed (y/n) that stood there with just as much shock. Then, just like that, Mordred was filled with anger. Perhaps she had been angry all the while, but seeing him after his death triggered it. Ritsuka, Fran, and Nursery Rhyme didn't hesitate to move out of Mordred's way as she walked up to (y/n) who continued to stare at her. Without thinking, Mordred swung her fist at him, but he off course dodged her anger induced attack.

        "Be still so I can hit you properly, you coward!" she snapped. "You can't just go and die like that!" Despite her order, (y/n) didn't stand still and let her hit him. Instead, he caught her fist to stop her entirely. "Let me go!" she growled. To her surprise, (y/n) did no such thing and instead hugged her.

        "Just calm down, Mordred." Hearing him speak stopped her outburst in that instant turning her anger into sorrow that she had never let go. "I haven't seen you in forever, so save the fighting for later, okay," he said.

        "You died. You weren't supposed to die..." Mordred mumbled, allowing herself to calm down in his hold.

        "Did you cry for me?" (y/n) asked with a smirk.

        "Hell no! Knights don't cry!" Mordred immediately denied and quickly wiped one of her eyes.

        "Um...do you two know each other?" Ritsuka asked. He along with Fran and Nursery Rhyme had been watching the whole ordeal in confusion.

        "I am a former knight from Camelot. I saved the kingdom a few times and stuff. Of course I know Mordred," (y/n) boasted. He attempted to give Mordred a few head pats which she rejected by pushing his hand away. "Are there any other knights here other than Mordred?" (y/n) asked. Before Ritsuka could say anything, Mordred spoke up.

        "Why do you care about any other knights? Aren't I the only one you wanted to see?" She slightly glared up at him since she was still trapped in his hold. It's not like she intened to escape any time soon anyway though.

        "You're not wrong...but there are a few of them I'd like to punch," (y/n) mused. "Why are you so worried? Did you get bored without a sparring partner to keep you company?"

        "You wish," Mordred grumbled before pushing him away and heading back to Ritsuka's room. "Come on, you've missed out on a lot here amd in Camelot. Might as well use this time to catch up on things," she added.

        "I'd also like to know more about you if you don't mind. I didn't know there was another knight of the round," Ritsuka stated.

        "That's because I'm not a knight of the round table. Seemed kind of boring to be dragged into meetings and things so I avoided it at all costs," (y/n) admitted. Mordred stopped paying attention to their conversation for a brief moment.

        "That idiot actually showed up. I didn't expect that...but I'm glad he did," Mordred thought. Her past was still her past, and her regrets were still her regrets. Even so, she was a servant of Chaldea now which meant she had to live in the present. Thanks to Chaldea's unexpected new arrival, that would be much easier now.
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Okay so I'm not sure if there was ever really a giant serpent that attacked Camelot...I made that up. Other servants included were Frankenstein and Nursery Rhyme.