Gundam Wing: Blind Target is also generally accepted as official. Blind Target was first broadcasted on Japanese radio, and then released as a set of two CDs. It was later edited and released as a series comprised of four comics in Japan.
Trowa and Catherine first appear in chapter "Target-01" on page fourteen. Catherine leaves the rest of the chores to Trowa when Ralph Kurt arrives. The two men talk. Pretty much there is little to no interaction there. If a person who believes in the brother and sister theory, it could be said that Catherine is acting like the older sister who makes the younger sibling do the work. It doesn't look that way in regards to her expression though. There is the likelihood that Trowa is just taking longer than usual because he wants to look after the lion and Catherine knows there is no point in rushing him. In any case, there appears to be no romantic hinting.
In "Target-02", it is learned on television that a terrorist bombing occurred at Colony Summit, where the colony representatives gathered to negotiate the problems involved with the entry into the union. Quatre is one of those representatives. Upon learning this, Catherine runs out and informs Trowa on page twenty-six.
When hearing this, Trowa looks briefly away from Kurt with surprise. Soon he regains his composure though and from Kurt's comment, he hints at being the part of the cause that did the bombing. Trowa glares at Kurt and once he leaves Catherine begins to catch on, especially when Trowa says he's an old associate. Quickly she grabs hold of Trowa's arms and voices how she doesn't want him to go away again. He gently takes her hands away and tells her he is sorry. He tells her,
"I don't want to be dragged into it. But I will surely return to you. Because this is where I can come home to."
The scene is quite powerful when it comes down to their closeness. Catherine knows the importance Quatre holds to him. She also can read his thoughts at these moments as well. Despite his methods are brief, Trowa in turn is gentle with her and reassures her. She lets him go as usual, because she understands him.
Like most scenes regarding his departure from the circus, there is never words of goodbye. It is a case of placing hope and trust in one another that Trowa will return. What a fan of the coupling would notice most is how he holds her hand when he says he is sorry and his choice of parting words.
Trowa doesn't detach himself by simply stepping away. He takes her hands and pulls them off gently and while telling her he is sorry his hands holds her within them. When they have pulled away from each other and Catherine watches Trowa walk off, her right hand is holding her left arm's wrist to her chest.
If this action is read into deeply, it could be considered romantic. Though it is only their hands there is again that gentleness and a sort of closeness that gets a person. When Catherine's hands are above her chest one could even read it as her hugging herself as she watches him leave worriedly.
The words impact the scene even more. With his simple apology and how he assures he will return to her rather than "all of you", it shows she is the most important to him. Even more, when he says "this" in regards to home, it can be read in two ways. It could be read that the circus is where he can come home to or that the relationship between them assures him he always has someone to come home to.
Those who insist they are siblings can easily argue that it just shows they are have a very close brother and sister relationship, though. It could likely work. Though Trowa is younger than Catherine, he often takes the protective and reassuring role when Catherine becomes quite distressed and emotional. Being both parentless and without any known siblings, it would make sense if they sought each other out in a sibling-like fashion and look out for each other.
In the end, while the scene was impacting in terms of their closeness, nothing really can be determined. Still, it does seem to show they are more than just friends. Just how more and in what way are the questions left to be answered.
In the last chapter "Blind Target After" Trowa returns home from his mission. While saying goodbye to Kurt on page 120, Catherine leaves a tent and sees them. She calls out Trowa's name. Seeing her, Kurt comments to Trowa,
"Go. That is the place you must protect."
When Kurt leaves, Catherine makes it over and welcomes Trowa home. She asks him if he's not hurt and he simply replies that he's home. A bit emotional, Catherine replies that is so like him but that it is good he made it back. Trowa smiles and notes he told her he'd come back for sure. She wipes a tear away and agrees. After commenting about Ralph as she sees him leave, she soon grabs hold of Trowa and tells him that the Ringmaster has been waiting. Trowa simply smiles.
Again, if anything, this shows just how important Trowa is to Catherine. One is able to see her concern for his well being and get teary-eyed over his return. In turn it is obvious to others that the circus troupe he travels with is what Trowa fights to protect. It is his home and thus his family. He smiles when welcomed back. Despite the emotions, you don't see anything that hints romance. There is obvious love, but whether it is romantic or platonic is still unknown. At best it shows there is the love of a family. What kind of "familiar" ties Trowa and Catherine have is not determined though.
The last time we see them together is on page 130. It only shows them performing their knife throwing act. Nothing more can be determined there.
The best information regarding the two comes from the Remix segments. In "Remix 2" on page thirty-nine. The following is a part of a translation from Viz Comics:
"2. TROWA: WHAT IS IT ABOUT THIS GUY?
He endures anything for the sake of the battle, speaks seldom and rarely betrays his emotions--it's easy to see him as little more than another surly teenager. And yet, Trowa is surprisingly popular with his elders; the real Trowa was devoted to the nameless boy (our Trowa), telling him all about his family, proudly showing him pictures of his sister and niece, and at the circus, Catherine treats him like a favored younger brother.
In the "Endless Waltz", Dekim Barton lets the boy into the army, even knowing him to be an impostor..."
There is also a caption under a picture of Catherine with her hand resting on Trowa's shoulder from "Episode Twelve: Bewildered Warriors." It says the following:
"Catherine fusses over Trowa as if she were his older sister. She can be pretty over protective."
While many fans would likely say that is undoubted proof, this is but an English translation and fans of show often know that context is often lost in translation. There is also the case of editors deciding what a relationship should appear to be before a young audience.
Also, it says nothing about blood ties, but if the wording is as faithful as possible to the true meaning, it does show Catherine does seems to see him like a brother more than anything else.
In the end, there is nothing to establish in Blind Target on just what their feelings are for each other in complete definition. Catherine at best seems to see him as family in an adoptive sense. With Trowa there only seems to be closeness, friendship and family-like feelings as well.