- Does Sigma discover his past? Who is he? (I know that Lambda is Mariandel, Maruken's sister, but I don't have a clue about Sigma) Why he can use the "Sword of Legend"?
He was cloned from from multiple genestructures part of which was DNA sample taken from a descendant of light, and Gizarofs entire plan was to create him to wield Langrisser. The body itself was probably given to him by Grove, as Gizarof is using the demon tribe to further his ambitions, you can see more on this in Langrisser 4, and the original body of what becomes Sigma is rumored to be related to Varna and Artimueller, but i doubt that, the art is simply similar, Urushihara's artwork almost ALL of it looks close to the same, visit a few of his fan sites and see.
- Rainforce's lackey, Zero (the guy who "kills" Omega in the first Scenario if you beat Rainforce). I thought he had betrayed Rainforce when he passes the Crimsonia barrier and locks Rainforce out; so, I was expecting a fight with Zero later, but he just appeared one more time (inside the Crimsonia barrier, I think) in a cutscene...
I believe his name is Aizel not Zero.. the guy with the slicked back grey hair and gold circlet on his head right? Anyhow yes he betrays Rainforce because he wants power, just like all lackeys, and gets blown to hell by Grove's cannon.
- Who is Grove (that slim sorcerer) and why Claret hates him? Is he related somehow with Sigma's past?
Grove is one of the head honcho's of the Demon Tribe, and with Boser disposed of he takes over, however he had to establish his supremacy against the Demon Tribe and succeeds in a short cutscene, eventually he plans to confiscate Alhazard from Rainforce's people so that he can be more powerful than Boser. He activates a bunch of mechanisms used to power that huge cannon that blasts down Rainforces ship that Aizel 'commandeers', I can't remember but I believe he has a very short time of having Alhazard in his posession but than you wax his ass and his bimbo to boot.All I know & remember about Claret hating him, is he jokes about him planning the death of her father, and I think he revives her Father's murderer (who's DNA and partial looks are put into Sigma) and Alfreds Fathers corpses, I can't remember who the second guy was so I'm not for sure but i really think it was the murderer, he had a pirate-like hat on.
- Who is Gaiel (the guy with white armor)? For what reason he forced Jessica to give him Langrisser and Alhazard? Why he isn't with Langrisser later (we find the sword inside a box...)?
Can't remember about Gaiel, but Langrisser is in a box because it was being secretly carried by air to its destination until the ship crashed carrying it, and everyone scrambled to find it. (And it also could be Career Soft being Lazy)
- Why Omega gets stronger near Langrisser? What happens on the last Scenario? Are they locking something with Omega's help? Why Langrisser must be used for that and why the sword appears broken after the credits (it certainly has something to do with "The End of Legend", right?)?
Most of this is already replied too, however he is wrong about Omega just becoming powerful out of nowhere, he SPECIFICALLY says that his power is increased significantly when nearing the Langrisser, and as a result to have the power to take it, and or find it (Ship crash level when its in a box)
but whats missing is that Langrisser and Alhazard being used later on, are CLONES of the Swords, Alhazard & Langrisser are sealed up or something close to that, but for emergency Gizarof had duplicates of Alhazard & Langrisser
and the clone Langrisser uses its remaining power to alter the ships course too stop it from A) Crashing into the moon and B) To change its orbit so that it doesn't come back for a long time.
- What happens to Rainforce after the last Scenario and to the characters in the epilogues
There isn't a way for me to tell you the epilogues as they change by how many times your commanders had to retreat in combat, what happens to Rainforce is very hastily explained that he will be put into cold sleep instead of killed, kind of like Jail but they leave him on the ship.
- What is that tree on the first Scenario?
I don't have a clue, i think it just has something to do with being filled with a high amount of spirit energy for whatever reason, I didn't bother looking into it.