He was too concentrated on his big brother to notice over the years, how frail physically and mentally his "Father" was becoming. When his "father" Uther was discussing with the Knights about an upcoming war against the Lich King, he got out of his hiding place and interrupted them :
"Father, let me go to war for you!"
"No, go to room, it's none of your business. War is an issue between adults."
"But Father, I am an adult now. I can go to war with the Knights." And the Knights at the round table all laughed at him, saying :
"A street urchin, going to war for his great Father Uther Pendragon?" Everyone in Camelot knew he was adopted of course, and some Knights even dared to disrespect him. But then again, he already experienced this when he was a child, so he held his head high and answered :
"A street urchin, but maybe more skilled in combat than you, my good sir Knight." This angered the Knight and he said in reply :
"You street urchin, how dare you mock me?"
"Because you mocked me first, my good Sir Knight."
At this, his "Father", in truth his biological grandfather said, to calm the Knight down :
"Knight, please, accept my son's apologies, he's only a child after all. Mordred, go and apologize to my Knight."
"But Father, he affronted me first, why should I apologize to him now?"
"Do as I said, Mordred. You're much more of hothead than your older brother Arthur." And to his "Father", he was less talented than him in everything : archery, jousting, and even horseback riding, and even had more success with the ladies, all popular amusements amongst the nobles' sons. He thought he should be jalous of his brother's talents, however he truly loved him, so he didn't mind his older brother besting him in everything. So whenever his "Father" commented about that, he'd think to himself : "He had more years of practice before me, so I can't even compare with him."
"His adopted brother." The knight whispered.
"What did you say, say it louder so I can hear?"
"Mordred, stop this now and go apologize to Sir Knight here."
"Yes, Father." And he did, although unwillingly and the Knights still mocked him by calling him a "street urchin" or "some bastard". But as his "Father" ended this round table discussion about the war, the Knights and himself retired to their respective room. He thought at night : "That Knight, I swear I'll defeat and humiliate him in a duel myself. He dared to mock me, Uther's son, although adopted, Father still treats me like a prince."
But he didn't have the chance to avenge himself, as the Knight who insulted him first fell in battle, as well as his own Father : Uther Pendragon. He saw a guard running towards the Castle, telling everyone :
"Uther is now dead!"
"Is it true, guard?"
"I wish it weren't true, your father was a Great King and war hero, but he was fatally wounded by Arthas."
"Arthas, but I thought he was a friend!" From the Castle, he saw Arthur rushing towards the guard.
"You're not telling me father's been killed by his most trusted trainee Arthas Menethil?"
"Yes the very one, your Highness! People say he turned against your father some while ago, and he has formed an alliance with the Lich King, that's how they won the war against your father."
"How, when and why?"
"Nobody's seen them duel, but people say Arthas challenged Uther to a one-on-one duel, and he mortally wounded your Father, your Highness."
"So my older brother Arthur is the King now, so he is your Majesty, not your Highness, guard, mind your mouth!"
"Yes your Highness Mordred." And the guard bowed to him and his brother before announcing the King's Death to the whole Castle.
Even though people like the guards or the maids at the palace called him "your Highness", he could feel they didn't want to call him that, as they believed he didn't have an ounce of royal blood. But ironically, he had the Pendragons' blood coursing through him, and was Uther Pendragon Lightbringer's illegimate grandson.
He watched the funeral held for his adoptive father, the Great King of Camelot with a broken-heart, just like his brother. The palace maids and lady-in-waitings all cried for him, though the guards and himself didn't. And neither did his good brother Arthur. After his real grandfather's death, Camelot was in Chaos, as many wanted the throne. He and his brother Arthur had many cousins and all desired the throne of Camelot, after the Great King's reign.
So hence why he proposed to every duke, and nobles and cousins assembled there, that the one who'd be able to extract Excalibur, the enchanted sword would be King of Camelot.
"Anyone who's able to retrieve the sword from the solid rock would be King of Camelot."
"Where is Excalibur, your Highness?"
"That I believe would be your task to find out while on the quest, your Grace."
"Yes Highness."
The ones who had a claim to the throne, one by one agreed with his proposition, and enthused, went to their horses at once, to find the legendary enchanted sword Excalibur. Since he, Mordred, the 2nd prince in his Father's house, had no desire to be King whatsoever, he stayed in the Castle, while his "Brother" and cousins went after Excalibur.
Since some of Castle's maids and guards thought he would be perfect for the throne, they asked him while serving him dinner:
"Why aren't you going, your Highness?"
"Might I remind you, I am just the King's adoptive son, I don't have any real royal blood in me." And he couldn't be wrong about that, since he didn't his true parentage : he was truly Uther Pendragon's grandson and Arthur's son, a Pendragon's direct descendant. But he was illegitimate, so even if he did know, he wouldn't have any claims to the throne of Camelot either.
"But your Highness, I still think you'd make the ideal King of Camelot." It was the maid whom he bribed to secrecy about him able to do magic when he was younger.
"No, I am not interested in the throne. I just want my brother to be a good king, and as loving as my late adoptive father was to the people. And stop arguing about this, my brother is the only legitimate heir to the throne of Camelot."
He had to be extremely firm about this, as some still objected to his brother Arthur becoming King of Camelot. He wanted to know why they did, so he asked straightforwardly :
"Why do any of you think I should be Camelot's King?"
"Because your brother Arthur is more arrogant and self-loving than you, your Highness. I fear he won't be looking after the people that much, like your father did." a random guard answered.
"He may be a little more arrogant than me, but his heart is good and I can assure you he'll put the people's best interests at heart." He said defensively. He always defended his brother no matter what, because when he was younger, he'd stand up for him if anyone insulted him or called him things like "street urching, a bastard".
"But, your Highness—"
"Stop, and this discussion is close."
Shortly after this intense discussion, the one he called "big brother" returned triumphantly from his quest for Excalibur. In fact, he was the only one amongst the claimers to the throne who could really pull Excalibur from it's rock. And everyone now, no matter how close they are in kinship to Uther Pendragon, acclaimed his "brother" now as he was crowned king, after his glorious quest. No one of their relatives dared to claim Camelot's throne afterwards.
"By the divine power given to me by God, I am now crowning Arthur as King of Camelot!" the Pope said.
"And I, Arthur Pendragon, now King of Camelot to serve it's people to my best."
"All hail, all hail the new King of Camelot : Arthur Pendragon!" People of Camelot in the crowd cheered and clapped as they said this.
The festivities after his "brother's" coronation lasted at least a week. Disgusted by the noblemen's over-stuffing themselves with foods and drinking too much and even gambling, flirting with the noble ladies, he turned away from the coronation's celebrations and he was in his own room now, at peace with himself. As someone quite solitary, he much preferred to be alone with himself, than these public activities.