inside of the girl's heart

Milliardo Peacecraft / Zechs Merquise

Milliardo joined OZ under the name of Zechs Merquise in hopes to find revenge on those that destroyed his country and murdered his parents. So because he sees himself has being a bloodstained soldier and no longer Milliardo Peacecraft, he thinks he is not fit to even call himself Relena's brother.

Zechs and Relena first meet in Episode Zero after he rescues her from a terrorist. It is because of him and the conversation they have together, that Relena comes to calls Heero a "prince of the stars" early in the television series. She doesn’t know who he is or what his face looks like, because he wears sunglasses during their encounter. But this works in Zechs’ favorite because has no intention to ever tell her that he is her brother.

Relena does not learn of Zechs’ true identity in the series until episode 16 during the battle in Antarctica with Heero. She yells over the radio for Heero to hurry and kill him (thinking he is only a former knight of the Cinq Kingdom), but Noin pleas for her not to wish for the death of her own brother. Relena is in complete shock at the knowledge of having a brother and no less him being a soldier in OZ. Though after a few minutes it all sinks in for her (maybe nothing can really surprise her anymore) and asks Noin if she’s in love with Zechs. When the battle concludes, it’s hard to tell if Relena is glad the battle is over for Heero or Zechs’ sake.

As the series progresses, Relena doesn’t seem concerned to worry at all about Zechs and her mind is constantly returning back to Heero and his well-being. Even when Zechs uses his birth name again has an ambassador of the Cinq Kingdom to the colonies, Relena easily disregards him to having any connection to the kingdom when she’s questioned by Duke Dermail. It is not until he takes the stage as the leader of White Fang as Milliardo Peacecraft, almost 24 episodes later after the battle in Antarctica, that Relena becomes concerned over his actions. Though her reasoning is not likely over the well being of a brother, but of the safety and future of the Earth that he has now sworn to destroy.

The relationship Relena and Milliardo though the course of Wing, is not one of happy and loving siblings. While on Libra, when Milliardo hugs Relena and comments on how much she has grown, she tells him almost bitterly that she doesn't even know if she is in the arms of her once kind-hearted brother Milliardo or the bloodstained hands of Zechs Merquise. To Relena, Milliardo is really a complete stranger. She doesn't know or understand anything about him, and comes to know Heero Yuy better than her own brother over the course of the show. Milliardo surprisingly doesn't seem to want any deeper relationship with his sister at the series’ ending and even into Endless Waltz. When he disappears after the war and is presumed dead, Relena is neither worried nor upset at his “death,” and it's not known if she even goes to visit his grave to pay respects. If anything, Relena holds deep respect for Milliardo, for being her elder brother and a former knight of the Peacecrafts, but how can she really love someone she knows nothing about as all, as a brother?

It can be noted in Blind Target that when Heero tells Relena that it's the remnants of White Fang behind the current attacks, she thinks of Milliardo right away with a look of sadness in her face. This can give a little hope in their relationship that Relena does feel remorse for brother.

At the closing credits of Endless Waltz, Milliardo is seen with Noin, leaving to help with Relena's Mars Terraformation Project starting in a few months.

Zechs doesn't have this "sister complex" that a lot of the fandom likes to portray in his character. He actually seems fine to let Relena go and do whatever she wishes, as long as she is safe and bringing peace to the world, as he believes she should. Even so, he seems to want to help her in the small way he can, without having to be near her. This is probably why he is willing to go off to Mars and help with her new project. The war is over and he really is a dead man walking. Milliardo Peacecraft has been dead since the day the Cinq Kingdom fell and Zechs Merquise died along with Treize, so he has no place on the Earth or in the colonies. To him, maybe his perfect place is far away from his home and sister and help her from there.

Their relationship is left pretty much open to interpretation at the closing chapter of GundamW. If a sibling relationship will start or nothing will change between them is hard to tell.