I'm only going to say that reading these descriptions most likely will spoil the series for you.  You probably saw it at least once in your childhood - that could be the only reason you are here now, so it really isn't a spoiler.  But, for those who haven't witnessed this great series, there may be a few plotpoints I will touch upon that really can piss off someone if they don't know about that in the series.  On that note, on with the descriptions.
Esteban
The star of the series.  His name translates to "Child of the sun", because of some odd ability to make the sun appear despite it being a cloudy day.  You're introduced to him as a young boy being taken care of by the resident churchhead, Father Rodrigez.  He's about 10-12 years old. Mendoza, a navigator that also is introduced in the first episode, had saved Esteban as a baby during an awful storm estimated about 10 years ago and took an important half of his sun engraved necklace as a memory.  There's a great deal more that I could delve into, but I'll probably just leave that for the episode summary page.  He's adventurous, energetic, determined, curious about his lost family (Mainly, his father), and a little afraid of heights.  A little made fun of in his group of three because he is the token 'Spanish kid'.  This, however, doesn't stop him from achieving his destiny. 
Zia
The secondary star of the series.  For all male characters, there is always a female counterpart.  Zia is introduced in cameo appearances in the first episode, and later properly introduced in the second.  She is the daughter of an Incan High Priest, although she doesn't let anyone really know that about her.  She fears that the Spaniards will use her to destroy her village should that information get lose.  Her overall goal is to find her way back home and escape the grasp of her kidnappers who use her incan knowledge for greed.  Esteban vows to always be by Zia's side in one of the early episodes.  This is just cute, because you can't really expect romance in this sort of series, especially between two ten year olds.  She, too, has a sun engraved necklace almost identical to Estebans (Save for the missing piece that Mendoza took), so she feels that there is a unique sort of fate that brought them together.  She doesn't trust the Spanish adults, and even occasionally lets her untrustworthiness of them get the better half of her in situations where trust is needed.  She is what I would affectionately like to refer to as "The Token Female".  There are no other mainstream women in this series save for one antagonist in the later half of the series.  Her goal is to find her father and return home.  She could care less about these cities of gold.

Tao
Found on some deserted far off island in what appears to be the Galapagos, Tao harasses our mighty primary and secondary castaway castmembers with the costume of an incan god and kidnaps Zia in episode 5ish.  You discover that he is the last of some ancient civilization known as the Hiiva (Hee-vah).  He appears to be a little older than the other two children, perhaps Twelve or Thirteen years old.  Again, it's never mentioned, but he has a noticeably deeper voice,  is much taller, and arguably wiser than the other two.  He brings with him a stick, a gold jar that cannot be opened, a book of Hiiva information that he can barely read, and a loving talking bird sidekick named Cocoapetal.  Tao is the missing key on a lot of levels, but could also be thought of as the comic relief, and the one that doesn't trust anyone.  His wisdom and ingenuity get the three through a great deal of situations.

Mendoza
A Spanish navigator who has aspirations to find the Cities of Gold.  He works under the Spanish army, and helped smuggle both Esteban and Zia aboard the Espiranza as stowaways to travel to the 'New World'.  Despite loyalties, he does the right thing on almost every occasion, normally in the childrens favor.  He saved Esteban when he was a baby in the middle of an awful storm, and so felt an obligation to him when Father Rodrigez passed on in episode one.  He offered Esteban a chance to sail to the new world, hopefully find this Father that may or may not be alive, and find the cities of gold.  He pops in and out of the series due to being separated from the children time and time again.  Although he is very important to the plot, I feel that Mendoza is somewhat a Major Secondary character while the children are the Main characters of the series.

Pedro and Sancho
Okay, these two are completely useless.  They suck.  They are the bumbling comic relief sidekicks of Mendoza.  Most of the time they are attempting to be good sailors, but in the end they are just complete morons.  I sometimes wish they drowned at sea early on so we wouldn't have to deal with their antics and screw-ups.  There are a few key sequences in the series that they do help, but for the most part they hurt more than help.  One looks like a monkey (Pedro), while the other is slightly overweight and stutters (Sancho).  I personally would shoot them on site if I ever saw them in person, despite being fictional.  Otherwise, they are very loyal and willing servants to Mendoza, though their greed shines through way too often to be realistic.  The series would have been better without them.

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