The odd one was the protector of Earth. Unlike all the other senshis, this guardian was a male. The Prince of Earth by birthright, HE was the leader and guardian of his birthplanet. He was.......

The anime did not show how Endymion and Princess Serenity, Queen Serenity's pure-hearted daughter, first met.
          But, in an episode where Kunzite (or rather Malachite) sent the senshis to another dimension, Queen Serenity's spirit seized the oppurtunity to enlighten the senshis... About their past-life in the Silver Millennium.
In that episode, Queen Serenity mentioned that Princess Serenity always gazed at Earth because she (the princess) loved someone there (you-know-who!).

                                          "Princess, I'm sorry I'm late!"
  "It's alright, Endymion!"
Suddenly, a guard noticed that someone was in the Royal Garden and roughly said," Who's that!", without knowing that it was Endymion.
                                        "Meet me in the hall, Princess!"

Endymion met his love, Princess Serenity, in the dancing hall, which was having a masquerade ball.
Dressed as Tuxedo Mask(!), he danced with Serenity.

Serenity asked what's with the disguise, and he quietly said, "There will be a great war between Earth and your kingdom. As the Prince of Earth, I'm not even supposed to talk to  you. Quick, alert your mother and her fighters. Beryl has been possessed and she has prepared the Earth people to destroy your kingdom!"
  Princess Beryl - A witch who had her heart set on Prince Endymion but was corrupted by Metallia's dark force.
 
Young, passionate lovers afraid for each other's safety.....

For the memory of their eternal love..

Serenity gave Endymion a musical locket...

In a moonlit night, two hearts unite as one...
 

Love like fire, nothing could separate them.

They kissed passionately... As though afraid it might be their last kiss...
                                                        

Before long, the war started. Beryl, Endymion's generals, and other weak-minded humans of Earth were all corrupted by Metallia.
Endymion left Serenity to try fend off the onslaught and convince his people that Metallia's just using them. (listen)

Seeing the vulnerable Princess Serenity, Beryl confronted the princess. Enraged that Endymion loved Serenity instead of her, Beryl prepared to kill Serenity. -"Ah! I shall silence your cute voice and make you a mute!!"-Beryl

Hesitating, Beryl seemed to ponder on Endymion's words. But then, a large, dark, shadow loomed behind Beryl. It was Metallia. Possessed, Beryl coldly replied,

With a full-scale war on the verge of erupting between their kingdoms, Prince Endymion promised Princess Serenity that he would ALWAYS protect her.

The war raged on, with the Moon Kingdom on the losing side. In a great blast, Endymion lost his footing and was being pulled away from Serenity...

"SERENITY!!"......  "ENDYMION!!!"
Unable to contemplate a  life without the other, the lovers started reaching out for one another.. Beryl saw this, and with uncontrollable rage, struck them with full power...
Killing Endymion and Serenity... 

Seeing the tragedy, and knowing that the Moon Kingdom was now without an heir, Queen Serenity knew that the time had come to use the Illusion Silver Crystal...

Sacrificing herself, Queen Serenity used the Silver Crystal to entrap Metallia, Beryl and slaves of the Negaverse. With her remaining strength, she sent her princess, Prince Endymion and the senshis to Earth to be reincarnated...

The queen KNEW that Endymion tried his best to stop his people from attacking her kingdom. Somehow, she also knew that he would ALWAYS protect her daughter, Princess Serenity. And with that came a flicker of hope. Hope that the Silver Millennium will rise again under the wise ruling of her daughter and the prince...