Lief means "heir"... and "loved", of which he is both.
In the original series, The Shadow Lord's minions The Grey Guards are everywhere. In the supplemental materials, the Grey Guards have been replaced by the golem-like Greers instead. In the Silver Door, it is revealed that the Shadow Lord is using a substance called jell to genetically manipulate a species called the Skimmer. Who's to say he didn't use the same process on the Greer?
All the first series main villains represent a different sin:
Gorl: Wrath, which pushed him into killing his brothers and ruining the forest.
Thaegan: Lust. Just look at how many children she has. Plus, cruelty is taking pleasure in the suffering of others.
Reeah: Vanity. It becomes his Fatal Flaw as he's too much busy gloating to notice Jasmine.
The Hive: Greed, obviously.
Gellick: Gluttony. He enslaved the Dread Gnomes, forcing them to farm flies for him and killing them if they couldn't replace the flies he ate.
The Glus: Sloth. It's a huge slug that can't move very fast, and it lives in a cave network where resting for too long will kill you.
The Guardian: Hubris. He's powerful and dangerous, but he believes that he's too great to be bested, so he plays games with his guests and eventually is defeated.
The Shadow Lord himself: Envy. He hates all "good" living beings, and wants to rule everything, especially Deltora, which was snatched from him multiple times. Plus, if he can't have it, well…
The Shadow Lord based most of his plans and traps on legends or things that actually happened. Considering that he faked a sister for Jasmine means that Anna might have been unknowingly pregnant when she was killed.
On another note, think carefully about the Ols. Supposedly, there's various tests to see if the people you think are on your side are really Ols, primarily the three-day test. But this doesn't work for higher than Grade 2 Ols. At any slow moment when Leif has his guard down and thinks the Shadow Lord's threat is over, the Ols can sneak in, replace Jasmine or Barda, and kill him off in his sleep. In a deeper level, how can you trust anyone? Even if they aren't Ols, the second plan of the Shadow Lord is mind control snakes. Anyone can be working for the Shadow Lord.
All of the mind control snakes were killed in the end of the second serial. Of course, the Shadow Lord can grow more, but without a pool of Deltorans he can send back to infest others, it'll be a lot harder to do anything with them. As for the Ols, it's a problem. But it's possible they can't even get into Deltora with the Belt at its full power and the Sisters defunct.
Doran's fate. Hundreds of years as the guardian of the Sister of the West, forced to exist as long as the Sister did, knowing that he failed and he'd never see his friends again.
The extent of Paff's miserable existence. She was one of the hundreds rescued from the Shadowlands, and returned home to… no family, no friends, nobody who actually liked or cared about her and a job with a fussy, hard to live with man who constantly compared her to his old assistant. Is it any wonder that she turned to the Shadow Lord? And even then it gets even more depressing, because he withdrew his powers from her at the last moment, leaving her to die alone.
Did Jasmine's parents just never tell her their names?
It's actually quite possible that they didn't, because if you're hiding in the woods, you wouldn't want to risk having a young child call out a name where anyone can hear it. So, if they only taught her to call them 'Mother' and 'Father,' it would both protect their identities in case she was captured by Grey Guards and keep her lineage a secret.
If the tribal leaders of the Masked Ones were actually descended from the bloodline of Ballum, King Elstred's younger brother, wouldn't that mean that Bede is Lief's heir, and not Marilen? A possible explanation for this plothole is that Ballum's line ended at some point, either through adoption or other means, but that would utterly mess up the explanation for why Bede looks like Lief in the first place!
Easy answer to that one - Marilen is said to be only one of several identified living relatives of Lief's - and thus potential heirs. He picked her out, that's all.
Even Easier answer- Marilen is said to be the decendent of Adin's second child out of five. This would place her far above the decendents of the last of Adin's children, and would make her second in line, especially if her mother is no longer alive. Due to lack of reference to her, that is something we can assume. So, in order for any of Ballum's decendents to become King/Queen, all the other decendents of all of Adin's other four children would have to die first.
Ballum's very existence causes problems, seeing as how it seems to contradict canon; remember, the reason that Lief had such trouble finding an heir was that part of the Rule was that the current King or Queen would have one and only one child. Adin was supposedly the only one who had multiple kids, which is why Lief had to trace his bloodline all the way back to Adin's time, and then down the bloodlines of one of his 5 original children. If any of the kings or queens had multiple children with the blood of Adin, his job would have been a lot easier - but if they didn't, that means that King Elstred couldn't have had a younger brother.
King Elstred was the oldest of Adin's five children, and by chance he only had one child. Ballum is Adin's youngest child, who was driven out of the castle by a rumor that he was planning on killing his older brother. So, Adin's second through fifth children had many children themselves, it was only the decendents of the first that had one. Because of this, Lief had to track down the decendents of Adin's second child, which was Marilen, which allows the cannon to still be in place, for the Masked Ones would come up as heirs to the throne, but a lot of people whould have to die before they could take over.
King Elstred and Ballum are Adin's grandsons, not his sons. Elstred and his daughter Adina started the tradition of the single heir. As for why Marilen was picked and not Bede? Yes, Bede is still technically closer in line, but the Masked Ones have hidden their relationship to the throne for centuries. Ballum's bloodline was probably thought extinct or untraceable. One final point: Lore says it must be worn by an heir of Adin. Not that the heir must be next in line. Presumably the Belt could jump around between heirs depending on circumstances, availability, and capacity to rule.
It is also possible that, while Ballum's descendants may or may not be closer in line by blood than Marilen, their exile and subsequent vendetta against the royal family has revoked their inheretence to the Throne and the Belt. It is stated in canon that the Belt and Lief's mask are opposing forces, so the Belt may well reject any of Ballum's ancestors, royal blood or no.
Why on EARTH didn't Sharn and Endon have more than one kid? If you have to have a blood heir to make your Macguffin work, ideally wouldn't you want as many as possible?
I think they mean why didn't Endon and Sharn have more children after learning that the Rule was bogus. Well, there are several possible reasons. (1) Simply not wanting another child. (2) Not being able to afford supporting another child on the modest living they were making from Endon's smithery. (3) Not wanting to bring more children into a world dominated by the Shadow Lord (they had conceived Lief while still blissfully unaware). (4) Not realizing that it doesn't have to be the first descendant in line in order to use the Belt. Actually, I don't think that was common knowledge, was it?
Also... If you're going to send that child on a round-the-country trip where Everything Is Trying to Kill You, you'd think a backup plan would be warranted, non? The reasons in the above entry do still hold, though.