I added that backstory because it's directly related to Isaac's quest and why he has to leave. The villagers of Vale were tasked with protecting Sol Sanctum, and protecting the Elemental Stars hidden within. Being a native of Vale, I figured the fact that those were hidden there was relevant, especially because the fact they are there is why Isaac's father is "killed" and why, three years after that, he has to leave to recover those.
I'll tweak a few of the other suggestions in the meantime though. Originally, I had Isaac first mentioned in the second paragraph, so I had to restructure it a bit for obvious reasons.
EDIT: Not mentioning them being unconscious as motivation only because they don't remember seeing the intruders. If they can't remember being knocked out by them, they can't really use that to motivate themselves, right?
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"If you're not here to help my friend, then get out of my way, so I can do it myself!"
In the world of Weyard, there are those known as Adepts, or those who can control one of the four elements. Venus represents the power of earth, Mars is fire, Jupiter is wind, and Mercury is water. Isaac happens to be a Venus Adept, and gets caught in a chain of events that threaten to change the face of Weyard forever. Much like the element he controls, Isaac is strong of heart with a slight stubborn streak. He's very kind-hearted, and will strongly trust those he counts as friends. Isaac also prefers to think things through instead of deciding hastily, examining all angles to ensure he's coming to the right decision. However, once he makes his decision, he'll see it through to the end, no matter what. Although he's a silent protagonist in the first game, he gains the ability to speak in The Lost Age. As for the reason for his journey...
Eons ago, the power of Alchemy, and with it many many Adepts, once flowed freely across the land. However, the ancients deemed Alchemy to be too dangerous for anyone to use and sealed it away using the Elemental Stars. These Stars slumbered within Sol Sanctum, located within Mt. Aleph. One night, intruders attempt to discover what was hidden there, and fail, triggering a trap which kills Isaac's father, along with the parents and brother of his friend Jenna. In the process, Isaac and friend Garet run into the survivors of the attempted raiders, and are swiftly rendered unconscious. Feeling as if those they lost could have been saved had they been more skilled with Psynergy, Isaac and Garet began studying the art heavily over the next three years. One day, fate leads the three to Sol Sanctum, where they discover something that triggers the start of Isaac's quest.
By the end of the events of the second game, Isaac is seen as one of the most powerful Venus Adepts alive. This strength is eventually passed on to his son, Matthew, who is the main character of the third game, Dark Dawn. By this point, both Isaac and Garet have taken up residence in a small cabin that overlooks Mt. Aleph, where they raise their children Matthew and Tyrell. It is here that Isaac helped to train the kids in both the arts of Psynergy and of battling with more traditional weapons; after an accident caused by Tyrell's headstrong nature, it's up to Matthew to start his own quest, one that takes him across the land and to the very edge of the world 30 years after his father's famous journey.
References:
http://goldensunwiki.net/Isaac
http://goldensunwiki.net/Vale