These are the most relevant relations for Eco. They contains a lot of spoilers, so be careful reading this section! :)

ECO AND BELCA

The relation between Eco and Belca is one of the most important of the story, because their journey is founded on it.
Eco saves Belca from death and it seems that he knows him very well, not just because Belca is the third prince, but because of a personal acquaintance,
In a first moment Belca doesn't like him, he thinks that Eco isn't affidable. But Eco saves him a lot of other times and Belca learns to consider him a friend.
Belca's distrust is owing to his situation and hip political position, he can't believe in no one. Knowing that someone killed his beloved brother Hector and that Orcelito and Musca are in danger, Belca tries to goes on alone. But Eco succeeds to persuade him that he can't do nothing alone.
Belca isn't sure of Eco's words because he can't understand WHY a person that he've never seen know is deciding to do a joureny together with him risking his life. But Eco's answer is that being a bard, he wants to sing the truth to the popular and stay together with him is find it in the deep of the country. So, he decides that his goal will be sing the song of Belca's life, because surely he'll be a hero!decide to do a journey together with the new friend in order to realize their dreams. Belca wants Eco wants
Belca isn't totally persuaded but he hasn't other chances.
Traveling with Eco Belca learns to trust in him and in many situations they save the life of the other one risking the proper one again and again.
Surely Eco's planes and Eco's real intentions aren't clear and he's hiding anything to Belca, but their bond is kind and I like their relation.

ECO & MUSCA

Usually Eco is kind with always, and obviousley he's kind with a child like Musca. In a first moment he appears like a prince to her, but knowing him, Musca understands that Eco isn't like he appears.

Their relation is formal but is cute look at them together because they share a lot of facial expressions and Eco is fearless to say to her the truth about the event and the situations of her dears, even if it could be cruel, as the time in which he was sure that Orcelito was dead and Musca insulted him with all herself because she can't believe that the beloved brother's life was finished.
When they turn back Sola Eco protects her in female clothes, and here we can see a funny and interesting thing: when Eco is transvestited in a girl he's very similar to Musca even physically, so we can assume that it isn't a simple coincidence.
Surely they share something more what Eco intendes to show to the others.

ECO & KILIKO

This relation isn't clear because in a chapter Kiliko shows some memories about a kid in his past, and that kid seems to be Eco.
A child that lived in the castle and that maybe is part of the royal family.
In the chapter 21 they share a meeting and Kiliko's face is hilarious. XD Surely they know the other one and Kiliko doesn't like him so much! XD