I thought I would go ahead and make a guide to prominent characters in The Legend of Zelda: Skyward Sword. Anyone who would like to add to this, please do so in the comments section, and I will make corrections and additions, as needed.

*** Warning: This contains spoilers to the storyline. ***

Demise:
He is the Demon Lord who brought an army against the ancient tribes of Hyrule before being imprisoned by Hylia. After Link destroys him in battle, he vows to return as a reborn soul, hatred incarnate, for eternity to battle the descendants of Link and Hylia.

Fi:
She is the personification of the Goddess Sword, created by Hylia to guide the Chosen Hero on his quest to defeat Demise, should the imprisonment one day break. Once the Goddess Sword transforms into the Master Sword and traps Demise, she goes into eternal sleep inside the sword.

Ghirahim:
He is the personification of Demise's sword, similar to Fi for the Goddess Sword. Once he uses Hylia's soul to break the imprisonment of Demise, he returns to his true form.

Groose:
He is the bully at the Knights Academy who eventually learns his place in the fight against Demise. Groose saves Zelda after she is tossed high in the air by Ghirahim and assists Link him fighting the Imprisoned form of Demise.

Impa:
She is a member of the Sheikah Tribe, whose members have sworn to protect Hylia. Originally from the past, she travels to the present to assist Zelda in learning who she truly is. Once Impa assures that Demise is destroyed in the past, she stays in the nearby temple to guard the remaining Gate of Time until she becomes an old woman.

Link:
He is Zelda's long-time friend in the Knights Academy at Skyloft. Link and all his descendants are foundlings that start as unlikely heroes but eventually take on the Triforce of Courage to counter the evil, reincarnations of Demise's soul, that threatens Hyrule.

Zelda:
She is the goddess Hylia in mortal form but does not remember her true nature until praying at three altars she made. Ages ago she gave up her immortal powers to imprison Demise, who brought an army against Hyrule in an effort to acquire the Triforce. Her final actions after imprisoning him were the raising of a chunk of land into the sky to protect the Triforce and the creation of the Goddess Sword that the Chosen Hero would one day use when the imprisonment broke.