[Thalion]

If you're sticking with the Silmarillion, then the two main characters are clearly and undoubtably Morgoth and Feanor...without these two, the tale doesn't happen, period, end of story...you need Melkor/Morgoth to steal the Silmarils and Feanor to swear an Oath which leads the Noldor back to Middle Earth...which is what the Silmarillion is about, the plight of the Noldor on Middle Earth...even if you include the Akallabeth, this is an extension of the plight of the Noldor (albeit mixed now with the plight of Men...but really they are called "half-elven" not "half-men") but now dealing with Morgoth's underling Sauron...

Back to point...Morgoth and Feanor...two main characters, that is what the Silmarillion is about...Oath of Feanor against Morgoth because of the Simarils, thats why the Noldor leave Aman

5 secondary characters is much tougher...

NOTE: Although I personally love the story and tale of Turin, his is the easiest removed from this discussion because he really never drives the Silmarillion story...he has no interaction with the Silmarills or with Morgoth...his story is named the saddest and he suffers most from the evil begot by Morgoth but not directly, only indirectly through trechery, sorrow and lies...therefore I think you can rule him out

1) Sons of Feanor...group these together (unless you want to do 6 plus Maedhros as he plays a somewhat more important role)...they as secondary characters are chiefly important because they continue the Oath of Feanor...although otherwise they suck because they don't do much for the story in terms of plots and interesting actions

2) Beren/Luthien...clearly important because a) they interact with the Silmarils, b) they interact with Morgoth (and his chief underling Sauron), c) they interact with the Son's of Feanor, d) their story was most dear to Tolkien's heart (his tombstone has Beren below his name and Luthien below his wife's name)

3) Earendil...the "Jesus" figure of this work...he not only is bound to a Silmaril (permanently in the sky), but he is the savior of Middle Earth when he voluntarily sacrifices himself to venture to Valinor for help

After those three, pick as you choose...Finrod helps Beren on his way...Tuor and his comming to Gondolin, which was one of the first stories Tolkien wrote and is the last stronghold of the Elves, minus Cirdan's havens of course...Hurin who is named Mightest of Warriors of mortal men, who interacted with Morgoth, Thingol, Turgon, ect...Hurin's tale more so than his son's is important because it ties together many plots...again, I love Turin, but his story is cheifly concerning the fall of Nargothrond and Glaurung, doesn't further the story of the Silmarils...

I also venture now to say that I would probably break up the Son's of Feanor into two groups...Maedhros, and the other 6 smucks...Maedhros deserves his own piece now that I think about it because he drives the plot significantly more than his brothers

anyways, those are my thoughts on the subject, all is well and open to debate...I'm sure many will disagree