Esmeralda is incredibly interesting and likable, even though she doesn't show up very often in the show. 

 

Esmeralda had a very hard life.  In the manga, she is a slave, sold to a farmer on Death Queen Island, where Ikki is training.  In the anime, she is the daughter of Ikki's deranged master.  I'm going to mostly go by the anime, as I have not read enough manga to use that version comfortably. 

 

Esmeralda obviously loves her father and Ikki both very much.  She tells Ikki that her father came back from Sanctuary changed, and she's not sure why.  Ikki knows that doing anything with her will cause both of them a ton of grief if/when the master (Guilty) finds out, and Esmeralda knows this too, but instead of arguing with Ikki, she simply keeps coming to take care of him.  She's smart enough to know that she's not going to win a logic argument with Ikki, and she might as well just go ahead and do what she wants to while Ikki can't stop her. 

 

Ikki has several flashbacks involving Esmeralda, especially when she took him to see flowers growing in a small field on the almost-desolate volcanic island.  Similarly, Esmeralda is a flowery field in Ikki's dead island. 

 

Ikki is a loner and likes to blame himself.  So when Esmeralda is killed by Guilty while Ikki is fighting him, Ikki blames himself.  To make up for his self-hatred, he first tries to turn his hatred on his master, but killing Guilty doesn't fix anything, so he turns his hatred on the world.  Eventually, he realizes what a beast he's being, and how he's completely betrayed Esmeralda and her gentle legacy, Ikki stops hating everyone.  Unfortunately, he goes back to hating himself.  Sometimes, when Ikki is floating on the edge of death, he hears Esmeralda talking to him, and telling him to get his butt in gear and go live and help his friends,  because death is the coward's way out.  And Ikki LISTENS.  Ikki may love his brother, but he doesn't listen to Shun most of the time.  Ikki LISTENS to Esmeralda.  This is not a small feat. 

 

Many people overlook Esmeralda.  I think one of the reasons I'm so not OK with Saint Seiya yaoi pairings is that the main four love interests are such likable and sweet characters.  Ikki obviously LOVES Esmeralda, and seems like the kind of person who would never ever ever look at any other girl ever again, just because he's still so stubbornly attached to her.  Everytime he's about to die, he thinks, awesome, I get to see Esmeralda again--that's how far he's gone. 

 

One of the things I really admire about Esmeralda is her ability to be so tough and sweet and uncomplaining even though her life really sucks more than anyone's.  Her father ignores her, she has to see the boy she loves get beat up every day, and she has no one else to talk to, besides Ikki.  She lives on a volcanic island, where the climate is so harsh that barely anything grows, and it's boiling hot every day and night.  But, somehow, Esmeralda doesn't despair.  She doesn't give up.  She keeps finding beauty and wonder in everything--it's Esmeralda who sees the goodness in Ikki, who finds the flowers, who is concerned for everyone.  And, she doesn't worry herself by thinking further than now, and those she loves.  Even when she dies, she doesn't cry about the unfairness to herself--she is STILL thinking of Ikki. 

 

I truly enjoy watching Esmeralda.  Ikki is mentally unhinged, lonely, and unable to communicate well with people.  Esmeralda, intuitive and strong and loving, fits him perfectly.